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This CL adds an invisible button which should trigger voice search.
Note that the UI will change significantly. The main purpose of this
CL is to add speech recognition API wrapper.

Bug: 773767
Change-Id: If760a88cc950b231d3b2c14b94403afd8f23c56b
Reviewed-on: https://chromium-review.googlesource.com/701174
Commit-Queue: Biao She <bshe@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510182}