Avoid some explicit WebSpeechRecognitionHandle() conversions.

It's now tidier to have these be implicit instead.

Also remove an unused WebSpeechGrammar() constructor + some Speech API
assignment operators.

R=
BUG=

Review URL: https://codereview.chromium.org/623953002

git-svn-id: svn://svn.chromium.org/blink/trunk@183242 bbb929c8-8fbe-4397-9dbb-9b2b20218538