Switches the Windows embedding from the standard C API to the new proctable version, to allow for unit testing of the embedding layer separately from the embedder APIs implementation. This includes moving some engine messaging that was still in flutter_windows to the C++ engine class to better encapsulate the proc table.

and roll Dart to 52783837369de45d3372cb6c6b7cdd63e71cd829.

Converts the macOS embedding to use the new proc table version of the embedding API, and adds one example unit test using it to demonstrate and validate the approach.

Bug:
  https://github.com/flutter/flutter/issues/67579

The framework handles arrow keys, delete, and backspace (and with better
unicode support), so we shouldn't handle them at the embedding level.

Fixes #69202

* logging

* fixing positions with wrong a11y borders screenreader-on/mobile browsers

* remove logs from the window class

* work on unit tests

* using reviewer suggestion for translations

* compute bounding matrix

* compute bounding matrix

* addding more comments

* reenable failing test case

https://dart.googlesource.com/sdk.git/+log/fed66f60a3bc..25ef5dc559cf

2020-10-30 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.12.0-4.0.dev

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