Port some rendering micro-optimizations from WebKit.

This ports http://trac.webkit.org/changeset/152212 and
http://trac.webkit.org/changeset/152213 into Blink.  These are trivially
correct and probably backed by profiling data.

This also deletes unused method updateDrawsContent().

NOTRY=true
BUG=None

Review URL: https://chromiumcodereview.appspot.com/18763007

git-svn-id: svn://svn.chromium.org/blink/trunk@154150 bbb929c8-8fbe-4397-9dbb-9b2b20218538