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Revert of Make ScrollbarThemeAura selectively invalidate scrollbar parts. (patchset #10 id:180001 of https://codereview.chromium.org/1525803002/ )

Reason for revert:
Oilpan unit test failure: http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac%20Oilpan/builds/28217

Original issue's description:
> Make ScrollbarThemeAura selectively invalidate scrollbar parts.
>
> In particular:
> - when hovering or pressing the thumb, repaint only the thumb
> - when thumb position changes, repaint the track only if it changes
>   (e.g. due to a button being disabled upon reaching the end)
>
> BUG=549277
>
> Committed: https://crrev.com/0e8396ada46f085c23343450d850d81751053c94
> Cr-Commit-Position: refs/heads/master@{#367082}

TBR=ccameron@chromium.org,skobes@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=549277

Review URL: https://codereview.chromium.org/1550983002

Cr-Commit-Position: refs/heads/master@{#367096}