Rename "CustomEvent::serializedScriptValue" to "CustomEvent::serializedDetail"

The "serialized script value" on CustomEvent is a serialization of the
 'detail' property. The setter makes this clear, but the getter makes
the code more confusing than it ought to be.

This patch simply renames the getter; there should be no behavioral change.

Review URL: https://chromiumcodereview.appspot.com/21099002

git-svn-id: svn://svn.chromium.org/blink/trunk@155094 bbb929c8-8fbe-4397-9dbb-9b2b20218538