Render opaque border with no shadow for web contents modal dialogs under
Views/Win32
This is a temporary kludge to get web contents modal dialog frames
displaying acceptably under Views/Win32. It's not possible to use
transparency in the frames on Win7 and earlier OSes unless the web
contents modal dialogs are top-level windows, which is undesirable.
Making web contents modal dialogs top-level imposes additional
complexities and burdens on managing focus, activation, and window
position relative to the browser window, soley for the View/Win32 case.
It's best to avoid this since we won't need it once we transition
to Aura.
See screenshots at
http://crbug.com/231012#c6.
BUG=231012, 166075