Fix border of constrained window button

The corner of the constrained window button looked too dark.

Fix was to not use black when drawing the shadow.

Screenshots:
  1x: http://i.imgur.com/R93jj4p.png
  2x: http://i.imgur.com/y4S3ZSq.png

BUG=292927
TEST=
  Navigate to http://www.pagetutor.com/keeper/mystash/secretstuff.html
  Verify that the corners of the Ok and Cancel button look good.
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/24365005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224989 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224989