Add a temporary engine systrace logger till the Dart timeline learns how to log to systrace. (#2978)

Noticed this while looking at https://github.com/flutter/flutter/issues/5594

runtime/BUILD.gn and ..core/BUILD.gn use flutter_runtime_mode which
is defined in common.gni. If the build comes through common/BUILD.gn
this is imported (implicity) but builds that depend on specific
files, like the Fuchsia build, won't have this imported.  This
imports the gni from the build files that use it instead of relying
on it already being there.

Don’t depend on tonic/debugger in release since the Dart debugger is unavailable in product mode. (#2972)

Add “—endless-trace-buffer” to switch to an endless buffer from a ring buffer while tracing. (#2966)

Make the URL loader more resilient to dropping the start callback on the correct thread in case of early connection termination. (#2964)

Since this functionality will be removed soon, this patch removes
the logging in order not to hinder the iOS development experience.

Pipelines vend single use continuations that callers can complete at any time to signal readiness of a pipeline resource. (#2957)

If the vsync provider is being replaced, make sure the animator expects no more signals from the old registration. (#2954)

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

This reverts commit 26f89708.

* Add config properties to specify snapshot blob file names.

This adds the ability of the shell to override the default
dart aot snapshot blob file names, and it wires up Android's
FlutterMain to recognize the properties in the app's manifest.

This will be used for flutter applications that build their
binary snapshots into files other than the default ones
that the engine uses.

Also, clean out some unneeded dependencies from engine/platform.

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

Will only be used by Linux Desktop for now.

This is now the canonical location of the Mojo public SDK.