Add support for layered windows in Aura
By layered meaning per-pixel alpha blended with desktop. We use
them in several popup bubbles, like html5 desktop notifications, the
app launcher, etc.
Under the right conditions we can trick the desktop window manager
to simply blend the D3D surface associated with the window and
the window manager desktop texture.
BUG=154069
TEST=see bug for examples
Review URL: https://codereview.chromium.org/
12255051
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@183353 0039d316-1c4b-4281-b951-d872f2087c98