Added dependency on CppWinRT for UWP builds (#24770)

CppWinRT is a tool that generates standard C++17 header-file libraries
for Windows Runtime (WinRT) APIs. A more detailed introduction to
C++/WinRT can be found in the Microsoft documentation at
https://docs.microsoft.com/en-us/windows/uwp/cpp-and-winrt-apis/.

The source code is available under an MIT license, from
https://github.com/microsoft/cppwinrt.

Bug: https://github.com/flutter/flutter/issues/70205

DEPS

@@ -558,6 +558,19 @@ deps = {
      'condition': 'host_os == "linux"',
      'dep_type': 'cipd',
    },
+
+  # CppWinRT tooling for UWP builds
+  'src/third_party/cppwinrt': {
+    'packages': [
+      {
+        'package': 'flutter/cppwinrt/win-amd64',
+        'version': 'build:2.0.210301.1'
+      }
+    ],
+    'condition': 'download_windows_deps',
+    'dep_type': 'cipd',
+  },
+
 }
 
 hooks = [

tools/cipd/cppwinrt/.gitignore

+# Temp directory into which we unpack the .nupkg archive
+cppwinrt
+
+# nupkg archives
+*.nupkg

tools/cipd/cppwinrt/README.md

+# CppWinRT CIPD package
+
+CppWinRT is a tool that generates standard C++17 header-file libraries
+for Windows Runtime (WinRT) APIs. These instructions describe how to
+update the CIPD package that bundles these tools for Flutter builds.
+
+A more detailed introduction to C++/WinRT can be found in the Microsoft
+[documentation](https://docs.microsoft.com/en-us/windows/uwp/cpp-and-winrt-apis/).
+
+The source code is available under an MIT license, from
+https://github.com/microsoft/cppwinrt.
+
+
+## Requirements
+
+Updating this package requires the following dependencies:
+
+1. [Depot tools](http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up).
+
+
+## Uploading a new CIPD package version
+
+To update the CIPD package, follow these steps:
+
+1. From the [CppWinRT package](https://www.nuget.org/packages/Microsoft.Windows.CppWinRT/)
+   page on nuget, click the _Download pacakge_ link.
+2. Copy the downloaded `.nupkg` file into this directory.
+3. Unzip the archive to a new subdirectory named `tmp`:
+   ```
+   unzip microsoft.windows.cppwinrt.<version_number>.nupkg -d tmp
+   ```
+4. `cd` into the `tmp` directory.
+4. Create the CIPD package:
+   ```
+   cipd create --pkg-def ../cppwinrt-win-amd64.cipd.yaml`
+   ```
+   The tool should output that the package was successfully uploaded and
+   verified, including the package path and an identifier SHA.
+5. Set a new `build:` tag:
+   ```
+   cipd set-tag flutter/cppwinrt/win-amd64 --version=<new_version_sha> --tag=build:<upstream_version>
+   ```
+6. Verify the package was successfully created and tagged:
+   ```
+   cipd describe flutter/cppwinrt/win-amd64 <new_version_sha>
+   ```
+7. Delete the archive and temp directory:
+   ```
+   rm -rf cppwinrt *.nupkg
+   ```
+
+
+## Updating the Flutter DEPS file
+
+Finally, we'll update the DEPS file to point to the latest version.
+
+1. Open the `DEPS` file in an editor.
+2. Locate the block covering `cppwinrt`.
+3. Update the `version` value to the version you just tagged.
+
+The block should look like this:
+```
+  'src/third_party/cppwinrt': {
+     'packages': [
+       {
+        'package': 'flutter/cppwinrt/win-amd64',
+        'version': 'build:<upstream_version>'
+       }
+     ],
+     'condition': 'download_windows_deps',
+     'dep_type': 'cipd',
+   },
+```
+
+Finally, re-run `gclient sync` to verify the package downloads
+correctly.
+
+
+## References
+
+* [CIPD for chromium dependencies](https://chromium.googlesource.com/chromium/src/+/67.0.3396.74/docs/cipd.md)

tools/cipd/cppwinrt/cppwinrt-win-amd64.cipd.yaml

+package: flutter/cppwinrt/win-amd64
+description: C++/WinRT tool used for building WinRT headers for Windows UWP builds
+install_mode: copy
+# The temp directory into which the nupkg archive is extracted.
+root: cppwinrt
+data:
+  # Contents of the cppwinrt/ directory are at the root of the CIPD package.
+  - dir: .