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This reverts commit 9e21c73c.

Reason for revert: suspect causing WebKit Android (Nexus 4) to fail on webkit_unit_tests
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/67957

Original change's description:
> Fix overlay scrollbar at huge scroll offsets
> 
> The overlay scrollbar ThumbPosition method was losing precision at very
> high scroll offsets. This would cause it to drift away from where we
> actually show the thumb from the compositor so the hit testing would be
> unreliable.
> 
> Looking at this method, there's no reason it should need to be any
> different from the base class ScrollbarTheme::ThumbPosition which is
> robust under high scroll offsets so the solution here is to simply
> remove the override. Less code, less bugs.
> 
> Bug: 739668
> Change-Id: Ib90152d989931cd0bcc0bddfb226d97d577ce575
> Reviewed-on: https://chromium-review.googlesource.com/598734
> Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
> Reviewed-by: Jeremy Roman <jbroman@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#491766}

TBR=bokan@chromium.org,jbroman@chromium.org,dtapuska@chromium.org

Change-Id: I44f227bc5956899ff43e7c6d96e7c5596666ae36
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 739668
Reviewed-on: https://chromium-review.googlesource.com/600848Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491798}