Make the animator host an implicit fallback VSync provider in case platforms cant provide their own. (#2953)

`VSyncProviderFallbackImpl` for platforms that lack the ability to provide real VSync callbacks. (#2941)

Will only be used by Linux Desktop for now.

* Update to mojo ba13534f2b2af27a1d73e176f7406dbab25f8e14

* Fix build for updated mojo

* Fix android build. Undo some of the mojo roll.

* Fix iOS build.

Updating base turns out to be more complicated than I thought.

This reverts commit daa822aa.
This reverts commit c38428c7.
This reverts commit 4b73a8a0.

This patch teaches Flutter how to connect to the vsync service on Modroid.
Also, remove our now-redunant copy of vsync.mojom.

Fixes #589

Now we actually use the vsync signal to trigger work. Previously, we'd hit the
pipeline depth limit too early and fall back to swap buffers-triggered
rendering.

Also, rename Vsync to VSync on recommendation from jamesr.

Instead of using back pressure from swap buffers to drive the engine, this
patch using the vsync signal from the Android framework. We still respect
back pressure from swap buffers if we get too far ahead.

Now that we have all the Dart packages we need in //third_party, we can build
skyx bundles by default.

As part of this change, I've made it possible to build skyx bundles on Linux
and I've made the gn target names of the mojoms in //sky/services consistent
with each other and with //mojo/services/public.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1227973002 .

This interface is growing to expose many activity-related functions, more than
just intents.

R=eseidel@chromium.org, eseidel@google.com

Review URL: https://codereview.chromium.org/1223053002 .

R=abarth@chromium.org, abarth@google.com

Review URL: https://codereview.chromium.org/1213623008.

We can use these intents to move the demo launcher to the new world.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1187583002.

This moves the definition of the mojom URLRequest and URLResponse
objects into the SDK at //mojo/public/interfaces/network/. These are
used directly by the shell and have a special place in the system. This
patch does the minimal amount of import/include updating to get tests
passing but doesn't fully update everything - in particular it
doesn't update all the C++ code that #includes url_loader.mojom.h for
the URLRequest or URLResponse definitions to #include the more specific
mojom.h files, or the equivalent in other languages. I'll do that in a
follow-up to minimize churn.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1129063008

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1018393006

This CL is the beginnings of an implementation of url_loader.mojom using the
OkHttp library. OkHttp is a relatively simple HTTP client library for Android
that implements HTTP/1.1 and HTTP/2.0. We might want to use OkHttp to bootstrap
into a more full-featured network_service implementation based on //net.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/930673002