Guard SpdyWriteQueue::Enqueue, and test all reentrancy points.

      New tests use SpdyBuffer callbacks to enqueue additional writes,
to verify that re-entrancy is allowed and that guards are not hit.

Also confirmed that reverting r268730 causes these tests to fail.

BUG=369539

Review URL: https://codereview.chromium.org/269423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269246 0039d316-1c4b-4281-b951-d872f2087c98