Handle zero-length strings correctly.

NewWebSocketChannelImpl::didReceiveData() treated empty strings as if
they were invalid UTF-8.

Fixed.

This fixes the timeout in the layout test http/websockets/frame-lengths.html.

BUG=342362
TEST=layout tests

Review URL: https://codereview.chromium.org/158903002

git-svn-id: svn://svn.chromium.org/blink/trunk@166978 bbb929c8-8fbe-4397-9dbb-9b2b20218538