Move keyboard::SendKeyEvent to ChromeVirtualKeyboardDelegate

This CL moves SendKeyEvent to ChromeVirtualKeyboardDelegate
which is the only place it is used.

It also:
* Uses aura::EventInjector instead of EventSink::OnEventFromSource so
  that events will be injected properly in Mash.
* Removes the undocumented histogram (without documentation it is very
  unlikely that it is used): VirtualKeyboard.KeystrokesBetweenBackspaces

Bug: 876138
Change-Id: I39836f5593587c32ccb229ca32a2126199500533
Reviewed-on: https://chromium-review.googlesource.com/c/1313208
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606899}
3 files changed