Combine ViewHostMsg_{Paint,Scroll}Rect into one IPC.

The combined IPC means that scrolling only requires one transport DIB instead
of two.  Previously, we'd use one in the ScrollRect IPC to pass up the pixels
for the exposed region, and then we'd use a second one in the PaintRect IPC to
pass up the pixels for the updated scroll bar rendering.  Now all paints are
done using a single transport DIB.

Optimize RenderWidgetHostViewWin::OnPaint to only paint the damaged regions.
This means calling GetUpdateRgn and GetRegionData to enumerate the list of
damage rects.  Then only those rects are copied from the backing store.

The same optimization is not done for Linux or Mac yet.

R=brettw
BUG=29591
TEST=none

Originally reviewed at http://codereview.chromium.org/506013

Review URL: http://codereview.chromium.org/509005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@35078 0039d316-1c4b-4281-b951-d872f2087c98

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