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Also, add padding around the tooltip text and disable the
border when shadows are enabled.

I'm also updating the existing shadow code in aura_shell to
clip corner images as needed when they're too large to fit
within the requested size.

BUG=106507
TEST=added


Review URL: http://codereview.chromium.org/8821028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113294 0039d316-1c4b-4281-b951-d872f2087c98