Enable composited scrollbars on Mac. 

Split ScrollbarThemeMac's paint function into separate parts when
possible, and call into ScrollbarThemeComposite to draw these parts.
Only do this when the overlay scrollbar methods are available.

Change ScrollbarThemeComposite to shrink scrollbars on overscroll, using
the same math as was removed from ScrollbarThemeMac.

Replace multiple instances of scrollbarMap()->get(scrollbar).get() with
calls to painterForScrollbar.

Call registerScrollbar on WebScrollbarThemeClientImpl scrollbars. If
this is not done, then ScrollbarThemeMac::minimumThumbLength will always
return zero because painterForScrollbar will return NULL for these
scrollbars.

Note that the animation of the scrollbars (fade-in, fade-out,
engorge-when-hovered-over, shrink-when-overscrolled) are all still
handled on the main thread, and lag behind the action that would
happen on the impl thread if input events were handled on the impl
thread. 

BUG=133093

Review URL: https://chromiumcodereview.appspot.com/21162004

git-svn-id: svn://svn.chromium.org/blink/trunk@155279 bbb929c8-8fbe-4397-9dbb-9b2b20218538