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This is largely a port of the existing X window manager's
code for drawing shadows around windows.  It adds an
ImageGrid class for drawing a scaled 3x3 grid of gfx::Images
and a Shadow class (managed by aura_shell::ShadowController)
that uses ImageGrid to draw a shadow.  Shadows can be
disabled via --aura-no-shadows.

BUG=101977
TEST=added unit tests; also manually checked that shadows get drawn

Review URL: http://codereview.chromium.org/8555025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111009 0039d316-1c4b-4281-b951-d872f2087c98