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The interval of last frame in sliding window is used to calculate the total expected frames in window.
Test are also updated to reflect this change in calculation of percent dropped frames.

Bug: 1167213
Change-Id: I5dd7e608d35b33af90046867d70bc652cced0366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633922Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844549}