Implement Composition interface for IME API

Spec: http://www.w3.org/TR/ime-api/#composition-interface
This interface represents an ongoing IME composition. It provides attributes
representing the text being composed by an IME and its associated information.

The interface IDL is already committed at r155072.
This CL implements the interface.

BUG=238585
TEST=layouttest fast/ime

Review URL: https://chromiumcodereview.appspot.com/23604053

git-svn-id: svn://svn.chromium.org/blink/trunk@157593 bbb929c8-8fbe-4397-9dbb-9b2b20218538