Revert 157593 "Implement Composition interface for IME API"

> 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

TBR=kochi@chromium.org

Review URL: https://codereview.chromium.org/24163002

git-svn-id: svn://svn.chromium.org/blink/trunk@157811 bbb929c8-8fbe-4397-9dbb-9b2b20218538