BackingStoreMac scrolling update and DCHECKs.
BackingStoreMac scrolling update and DCHECKs.

Functional change: when a CGBitmapContext is in use instead of a CGLayer,
scroll.  CGLayerGetSize(NULL) was just returning (0.0, 0.0), which was causing
no scrolling.

Functional change: avoid entering the scroll blitter if:
(0 < abs(dx) < size_.width() and abs(dy) >= size_.height()) or
(0 < abs(dy) < size_.height() and abs(dx) >= size_.width())

Non-functional change: DCHECK in a few spots that exactly one of a CGLayer and
a CGBitmapContext is present.

BUG=26989
TEST=Things should paint when scrolling.
Review URL: http://codereview.chromium.org/376028

git-svn-id: http://src.chromium.org/svn/trunk/src@31493 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 21c03e5134356ff3ba87f5ee7f816b7d9d2aada1
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser