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After commit crrev.com/917ce177
introduced the AverageLagTrackingManager some cc_unittests started
failing.
These test tested scenarios where frames were Drawn but didn't receive
a PresentationFeedback on LayerTreeHostImpl. They were failing due to a
DCHECK at  AverageLagTrackingManager that checked the amount of pending
frames without feedback before destruction.

These frames never received a PresentationFeedback because
LayerTreeFrameSink was purposefully losing its context
(LayerTreeFrameSink::OnContextLost).
This prevents DisplayScheduler to post the tasks that will ultimately
call LayerTreeHostImpl::DidPresentCompositorFrame.

In this CL the list in AverageLagTrackingManager is cleaned whenever
LayerTreeFrameSink loses its context, as it is a sign that the current
pending frames will never receive a presentation feedback.
Also, frames that don't have relevant latency_info aren't added to the
list anymore as this was one of the reasons why the tests where flaky
too.

Bug: 1103289, 1104841, 1106020, 1104202
Change-Id: I9b74ac22adb6db6af7c8e727304dc2e0b02e8899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303363Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: João Victor Almeida de Aguiar <joalmei@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#791413}