SPDY: Add a bunch of CHECKs into SpdyWriteQueue to detect re-entrancy.

      Specifically only look for re-entrancy that shrinks the queue. We know
that other types of iterator invalidation happens due to growing the
queue, but that doesn't match the symptom we're seeing.

BUG=369539

Review URL: https://codereview.chromium.org/265093004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267984 0039d316-1c4b-4281-b951-d872f2087c98