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Some of the stories (e.g. canvas_*_pixels_per_second,
text_*_pixels_per_second, etc.) scroll the page, but depending on the
velocity (e.g. when velocity is over 20000 pixels/second), it can end
too quickly, and produce only a handful of frames. As a result, some of
the metrics (e.g. PercentDroppedFrames.AllSequences) are not reported,
since they require at least a minimum number of frames. This can result
in some flakes (e.g. crbug.com/1113556) in the dashboard.

To fix this, keep scrolling the page for at least 15 seconds, so that
there are enough frames produced to report the metrics.

Bug: 1141854
Fixes: 1141854

Change-Id: Ia6adb0257081bf2d321c62e2c362ece3fe4c9f49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495340
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Mingjing Zhang <mjzhang@chromium.org>
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820845}