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This allows tracking the scrollTop of the page.

BUG=1109035
TEST=covered by tests

Change-Id: I34dd29cacb2569704176d8d3027347cdbd9be734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422650
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809596}