This patch factors out the nine-patch generation from NinePatchLayer
into a NinePatchGenerator and uses it to create a nine-patch scrollbar
layer that can change size without a repaint. This is used only in the
compositor. Scrollbars painted in Blink paint to the full required size.

The new scrollbar type is implemented in OverlayScrollbarLayer and Impl.
We add methods to NativeTheme to draw 9-patch versions of a theme part,
currently implemented for the scrollbar thumb for overlays in
NativeThemeAura. The NativeTheme provides two methods:

NinePatchCanvasSize() returns the size of the smallest canvas onto which
a nine patch part can be drawn.

NinePatchAperture() returns the rect in the canvas, whose size is
provided by the method above, which will be used as the middle patch in
the resource. Resizing the part will stretch only the middle patch in
both directions.

This is used mainly for the thinning animation in Aura overlay
scrollbars. We still need to repaint due to hover and pressed effects.
Technically, we could avoid repainting on size changes too but this would
require some additional work.

BUG=669670
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2591863003
Cr-Commit-Position: refs/heads/master@{#454413}