Switch to transparent, overlaid omnibox shadow.

Chrome OS previously drew a shadow with an opaque background
onto the omnibox and then rendered text with a transparent
background on top of it. The text's transparent background
resulted in subpixel rendering being disabled.

So that subpixel rendering will be used, give the text an
opaque background and draw combined border/shadow images
(with transparent backgrounds) on top of everything else.

Note that this results in shadows being drawn on all
platforms.

BUG=233364
TBR=oshima@chromium.org

Review URL: https://codereview.chromium.org/117753003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242572 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242572