Add app.window.alphaEnabled() and onAlphaEnabledChanged.
This allows an app to determine whether a window created with
"transparent_background" will work as expected.
This also allows the app to detect when "transparent_background" might
stop working, e.g. when Windows changes from Aero to Classic.
This also fixes the bug where "transparent_background" windows
created in Classic render as black rectangles.
BUG=260810
Review URL: https://codereview.chromium.org/
375183002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284026 0039d316-1c4b-4281-b951-d872f2087c98