UMA: Count the number of WebSocket.send() call for each argument type.

For the initial version, we just check distribution over all
connections. Not counts during each connection.

BUG=128156,359926

Review URL: https://codereview.chromium.org/244923002

git-svn-id: svn://svn.chromium.org/blink/trunk@172595 bbb929c8-8fbe-4397-9dbb-9b2b20218538