FakeXKeyboard is used in dev and test environment by default.

MockXKeyboard renamed to FakeXKeyboard. FakeXKeyboard is used in dev
(desktop) and test environment by default.
This change is needed to improve testability of following changes in
chromeos::Preferences class.

TBR=nkostylev,nona

BUG=325564
TEST=none

Review URL: https://codereview.chromium.org/134433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250711 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250711