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This fixes an assertion failure when trying to use this class on the Mac due to SkBitmap destruction happening during process exit handling.

BUG=None
TEST=Custom startup page list should still show default favicons.

Review URL: http://codereview.chromium.org/3028030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@54014 0039d316-1c4b-4281-b951-d872f2087c98