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- Add FrameSequenceTrackerType::kCustom;
- Add FrameSequenceTrackerCollection::StartCustomSequence to create
  a kCustom typed tracker to represent a sequence;
- Add FrameSequenceTrackerCollection::StopCustomSequence to schedule
  termination of a tracker for a sequence;
- Data collected by custom trackers are reported via the return
  value of FrameSequenceTrackerCollection::TakeCustomTrackerResults;

Bug: 1021774
Change-Id: I62d9c2eb833e1d40e01b3b452d515ce361233a64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007937
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754680}