Use null Strings for default values of MessageEvent properties

Use null Strings for default values of MessageEvent properties instead of empty
ones. Null strings are cheaper to construct and there is no web-exposed behavior
change since a null String is exposed as an empty one on JS side.

This change is based on WebKit r155959.

R=haraken
TEST=fast/events/constructors/message-event-constructor.html
BUG=

Review URL: https://chromiumcodereview.appspot.com/23591059

git-svn-id: svn://svn.chromium.org/blink/trunk@158113 bbb929c8-8fbe-4397-9dbb-9b2b20218538