Never send empty non-final messages to Blink.

The IPC message OnDataFrame() in websocket_messages.h documents the
restriction that frames must be either non-empty or final. Up until now,
net::WebSocketChannel has permitted empty opening frames through. Fix
it.

BUG=343060
TEST=net_unittests --gtest_filter=WebSocket*, fragmented-binary-frames layout test

Review URL: https://codereview.chromium.org/171813014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252644 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252644