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So that we can avoid creating PassThroughFilter.
The CL also makes FilterChain::GetHead() a little more efficient (SyncDispatcher calls it once for receiving each message).

BUG=None
TEST=None
R=darin@chromium.org

Review URL: https://codereview.chromium.org/291583005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271107 0039d316-1c4b-4281-b951-d872f2087c98