Revert of Implement overlay scrollbar fade out for non-composited scrollers. (patchset #7 id:120001 of https://codereview.chromium.org/2467693002/ )

Reason for revert:
WebKit Linux Precise MSAN failing since
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Linux%20Precise%20MSAN/builds/855

Errors like

STDERR: ==4==WARNING: MemorySanitizer: use-of-uninitialized-value
STDERR:     #0 0xafdca9e in showOverlayScrollbars third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp:569:7

Original issue's description:
> Implement overlay scrollbar fade out for non-composited scrollers.
>
> In crrev.com/2442573002 and crrev.com/2453553003 I added fade out for Aura
> overlay scrollbars. In this patch I add a simple instantly disappearing
> fade-out for scrollers that aren't composited.
>
> The changes in this patch are:
>
> -Plumb through the fade out durations into Blink. Any time a ScrollableArea is
> scrolled or resized we show the scrollbars and start a timer to make them
> disappear. The scrollbars are marked as "hidden" and made disabled and invisible
> to hit testing.
>
> -Fixed hit testing for overlay scrollbars so we can't scroll by clicking on the
> track (which isn't painted).
>
> -Fixed two layout tests:
>   third_party/WebKit/LayoutTests/fast/scrolling/scrollbar-tickmarks-hittest.html
>   overlay-scrollbar-mouse-capture.html
>   These tests turn on overlay scrollbars but try to scroll by clicking a
>   button/track which I fixed. The tests now drag the thumb instead.
>
> -In crrev.com/2453553003 I added didChangeScrollbarsHidden to disable overlay
> scrollbars so they're invisible to hit testing. It turns out Mac overlay
> scrollbars already have this functionality in
> ScrollableArea::scrollbarVisibilityChanged and
> Scrollbar::shouldParticipateInHitTesting so I removed didChangeScrollbarsHidden
> and used these. I also removed some redundancy in the Mac path so that Mac and
> Aura overlays disable hit testing in the same way.
>
> -Removed the static compile assert about ScrollableArea staying small. There's
> no reason ScrollableArea is particularily special and this encourages
> developers to duplicate functionality in the descendant classes.
>
> BUG=592098
> CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
>
> Committed: https://crrev.com/4430234f2f5f52efe60ad2b8e6608112352259da
> Cr-Commit-Position: refs/heads/master@{#429616}

TBR=jbroman@chromium.org,dtapuska@chromium.org,piman@chromium.org,pkasting@chromium.org,rbyers@chromium.org,skobes@chromium.org,bokan@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=592098

Review-Url: https://codereview.chromium.org/2478463003
Cr-Commit-Position: refs/heads/master@{#429766}