Move paint code for RenderScrollbar and RenderScrollbarPart into ScrollbarPainter.

Move paint code for RenderScrollbar and RenderScrollbarPart into ScrollbarPainter.

In so doing, exposed a new getPart() method on RenderScrollbar to get the part.
Also, this patch changes the code in RenderScrollbarPart to explicitly call the BlockPainter::paint method, rather than relying on virtual function calls. This is possible because RenderScrollbarPart is a FINAL class that inherits from BlockPainter. Therefore all of these calls must be the BlockPainter version.

BUG=412088
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182855

[chrishtr, 2014-09-27 00:34:23]

chrishtr@chromium.org changed reviewers:
+ pdr@chromium.org

[chrishtr, 2014-09-27 00:34:50]

chrishtr@chromium.org changed reviewers:
+ schenney@chromium.org

[pdr., 2014-09-29 17:34:41]

On 2014/09/27 00:34:24, chrishtr wrote:

LGTM. Sadness about that layering violation :(

[pdr., 2014-09-29 17:34:45]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2014-09-29 17:34:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/613503002/1

[commit-bot: I haz the power, 2014-09-29 17:38:57]

Committed patchset #1 (id:1) as 182855