Add ShapedScreenCapturer

ShapedScreenCapturer is a layer on top regular screen capturer that captures
desktop shape and attaches it to the generated frames.

R=wez@chromium.org

Review URL: https://codereview.chromium.org/93973006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241614 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241614