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, 662402
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Committed: https://crrev.com/4430234f2f5f52efe60ad2b8e6608112352259da
Committed: https://crrev.com/60d2bbbc8fa4cce01bbd90c97d3e989a47d6dedf
Review-Url: https://codereview.chromium.org/2467693002
Cr-Original-Original-Commit-Position: refs/heads/master@{#429616}
Cr-Original-Commit-Position: refs/heads/master@{#430065}
Cr-Commit-Position: refs/heads/master@{#431466}