Add routine for checking startTime when start the next utterance in handleSpeakingCompleted()

Same functions are called twice when calling speak() function repeatedly like current issue.
This issue is caused by asynchronous call.

So, I add routine for checking startTime in handleSpeakingCompleted() that is for preventing call twice.

BUG=367535

Review URL: https://codereview.chromium.org/398683005

git-svn-id: svn://svn.chromium.org/blink/trunk@178344 bbb929c8-8fbe-4397-9dbb-9b2b20218538