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Traces collected through Slow Reports show that acquiring the
GpuChannelHost lock from threads running at different priorities can
cause priority inversions. In particular, it can cause jank on the
renderer main thread through this chain of dependency:

> Main thread: cc::ProxyMain::BeginMainFrame
  > Compositor Thread: TileManager::FlushAndIssueSignals
    > CompositorTileWorkerBackground (slow):
        OneCopyRasterBuffer::Playback

This CL adds trace events to make contention on the GpuChannelHost lock
more obvious in traces, and an assertion to ensure that the
GpuChannelHost lock is never acquired at background thread priority.

Bug: 1072756
Change-Id: I2a25efe2e3f6e9bf5d980ca351a990f9c7d256b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157659
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823744}