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This CL enable the reply button on the right of the textfield in the inline reply.
So, the message in the textfield can be submitted not only by pressing enter key but also clicking the reply button.

TEST=message_center_unittests
Bug=803759

Change-Id: I6e7518d0fabd329d52d589b2024357869aca2fa2
Reviewed-on: https://chromium-review.googlesource.com/885693
Commit-Queue: Megumi Hattori <megumihattori@google.com>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532345}