Update IME API interfaces to the latest spec.

This is based on this revision of the editor's draft spec:
https://dvcs.w3.org/hg/ime-api/raw-file/9d03590d2137/Overview.html

BUG=264593
TEST=LayoutTest (fast/ime/*, 2 tests)

Review URL: https://chromiumcodereview.appspot.com/20676002

git-svn-id: svn://svn.chromium.org/blink/trunk@155072 bbb929c8-8fbe-4397-9dbb-9b2b20218538