Use ui::KeyEvent for ProcessKeyEvent.

This patch stops using IBus specific key events, but use the common ui::KeyEvent class.  This will enable to merge handlers of native events and fabricated events.

BUG=312218

Review URL: https://codereview.chromium.org/88313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237838 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237838