Clear |composing_text_| after CommitText() is called.

|composing_text_| of InputConnectionImpl should be cleared after
CommitText() is called. Otherwise, FinishComposingText() will commit the
same text twice.

Bug: 899736
Test: unit_tests
Change-Id: Idb22d968ffe95d946789fbe62454e8e79cb0b384
Reviewed-on: https://chromium-review.googlesource.com/c/1304773
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603518}
  void Reset() {
    ui::MockIMEInputContextHandler::Reset();
    send_key_event_call_count_ = 0;
  }

  void Reset() {
    ui::MockIMEInputContextHandler::Reset();
    send_key_event_call_count_ = 0;
  }
