More IME bugs.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c27b7317d4d4a457a51d589efcf2f79797071c66

commit c27b7317d4d4a457a51d589efcf2f79797071c66
Author: Jani Hautakangas <jani.hautakangas@lge.com>
Date: Tue Nov 28 07:56:40 2017

IME for Mus: Do not forward calls from unfocused text input client

In some cases calls to IME can come from unfocused client.
MUS IME as has a remote text input client which is set focused
when IME session is started. If a call from unfocused client
comes after IME session has been started then that call on
IME service side is interpreted to originate from currently
focused client which is wrong and can cause unwanted states.

Example use case where things go wrong is
Embedded Search API searchbox on NTP.

1. Open Chrome browser
2. On Chrome's initial google page click searcbox
3. Type something on IME or on keyboard
4. Focus switches to addressbar omnibox and TEXT_INPUT_TYPE_NONE
   is sent to IME from unfocused client.
5. IME running on IME service side interprets this TEXT_INPUT_TYPE_NONE
   to come from currently focused remote text input client.

Bug:  672585 
Change-Id: Ib5f5e6f73594580cb59fe966b52cbe5efb37ea79
Reviewed-on: https://chromium-review.googlesource.com/786232
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519601}
[modify] https://crrev.com/c27b7317d4d4a457a51d589efcf2f79797071c66/ui/aura/mus/input_method_mus.cc
[modify] https://crrev.com/c27b7317d4d4a457a51d589efcf2f79797071c66/ui/aura/mus/input_method_mus_unittest.cc
[modify] https://crrev.com/c27b7317d4d4a457a51d589efcf2f79797071c66/ui/aura/test/mus/input_method_mus_test_api.h

Issue 637418 is about whether InputMethodMus should handle OnInputLocaleChanged(). OnInputLocaleChanged() not used in chromeos. Thus not relevant to mash, IMO. Removing it as blocker and close this bug.