Fix DCHECK in WebSocketChannel::SendFlowControl

Debug builds would crash in WebSocketChannel::SendFlowControl due to an
overly-strict DCHECK(). Relax it.

BUG=327608
TEST=net_unittests --gtest_filter=WebSocketChannel*

Review URL: https://codereview.chromium.org/112663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240261 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240261