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This is needed because there can be two site instances for the
same extension/process pair. Boo.

The linear scan for Contains() is a bummer. I thought about
more exotic data structures, but it didn't seem worth it.

BUG=105328

Review URL: http://codereview.chromium.org/8769022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112921 0039d316-1c4b-4281-b951-d872f2087c98