Add FrameView::DeferredRepaintScope

Add an RAII for deferring repaints and use it whever we do this. Not pairing
these calls would be bad and some of the code is pretty hard to verify
the correctness of right now because we have so many paired calls all over
FrameView. This starts to clean that up.

Review URL: https://codereview.chromium.org/25847002

git-svn-id: svn://svn.chromium.org/blink/trunk@158911 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move webaudio module from wtf threads to WebThread API

BUG=301514
R=abarth@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/25232002

git-svn-id: svn://svn.chromium.org/blink/trunk@158912 bbb929c8-8fbe-4397-9dbb-9b2b20218538