compositionstart event's data should contain text to replace.
spec:
http://www.w3.org/TR/DOM-Level-3-Events/#event-type-compositionstart

As far as I tested, IE10 on Windows7 and Firefox22 on Linux
conforms the spec's behavior about event.data of compositionstart event.

BUG=247878
TEST=layoutest

Review URL: https://chromiumcodereview.appspot.com/18041002

git-svn-id: svn://svn.chromium.org/blink/trunk@153319 bbb929c8-8fbe-4397-9dbb-9b2b20218538