Merge 284026 "Add app.window.alphaEnabled() and onAlphaEnabledCh..."

Merge 284026 "Add app.window.alphaEnabled() and onAlphaEnabledCh..."

> 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

TBR=jackhou@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285044

[amineer_google, 2014-07-23 21:24:10]

Committed patchset #1 manually as r285044.