Add yhanada as OWNER for ChromeKeyboardUI

ChromeKeyboardUI used to be a trivial class that implemented
KeyboardContentUI. Because it was the only implementer, these two 
classes were flattened into one in crbug.com/739083
However, this inadvertently caused us to lose OWNERS control over this
code.

Bug: 
Change-Id: I980ef970f1a8467e7f8dcc0a675068d8b6d22a00
Reviewed-on: https://chromium-review.googlesource.com/720718Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509643}

chrome/browser/ui/ash/OWNERS

@@ -5,5 +5,6 @@ sky@chromium.org
 stevenjb@chromium.org
 
 per-file session*=xiyuan@chromium.org
+per-file chrome_keyboard_ui*=yhanada@chromium.org
 
 # COMPONENT: UI>Shell