cc: Don't explode on SingleThreadProxy::SetDeferCommits

This is two-sided prerequisite for
https://codereview.chromium.org/134623005/.  Once this lands, then
SetDeferCommits can be called regardless of threaded mode in
WebViewImpl.  This patch will make that not explode on NOTREACHED.

BUG=none

Review URL: https://codereview.chromium.org/380853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282173 0039d316-1c4b-4281-b951-d872f2087c98

cc/trees/single_thread_proxy.cc

@@ -201,8 +201,6 @@ void SingleThreadProxy::SetNextCommitWaitsForActivation() {
 }
 
 void SingleThreadProxy::SetDeferCommits(bool defer_commits) {
-  // Thread-only feature.
-  NOTREACHED();
 }
 
 bool SingleThreadProxy::CommitRequested() const { return false; }