An improved fix for TTS Extension API problems on Windows.

An improved fix for TTS Extension API problems on Windows.
The Windows speech API doesn't have a Stop function, and
we were using Pause before to implement this, but this meant
the utterance was still around, causing events from a
previous utterance to still get sent when speech was
resumed again.  Instead, now we call Speak with an
empty string and cause it to flush the speech queue. This
ensures that only one utterance is active at a time.

BUG=94466
TEST=manual testing with several talking extensions
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=99015

[Ben Olmstead, 2011-08-30 20:55:43]

LGTM