Fix composition key-code after switching to a new input field.

A line of code that resets the composition state upon
switching to a new field got removed by mistake during the
clean-up of dead code in the last CL.  The adds it back and
creates a test to make sure the switching works.

BUG=118639

Review URL: https://codereview.chromium.org/411383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286510 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed