Share code between ScrollingCoordinator and FrameView that were running the same
algorithm to find paint invalidation containers across frame boundaries.

Put the shared method on RenderLayer, where it makes more sense.

BUG=384520

Review URL: https://codereview.chromium.org/355843003

git-svn-id: svn://svn.chromium.org/blink/trunk@177326 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Prefer synthetic bold/italic over alternate family

Change the font fallback code to prefer the specified font family with
syntetic bold and/or italic over falling back to an alternate font.

R=pdr@chromium.org
BUG=385069
TEST=fast/text/font-fallback-synthetic-italics.html

Review URL: https://codereview.chromium.org/361833003

git-svn-id: svn://svn.chromium.org/blink/trunk@177327 bbb929c8-8fbe-4397-9dbb-9b2b20218538