Refactoring: Add Event::createBubble.

This reduces usage of bool arguments.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23714003

git-svn-id: svn://svn.chromium.org/blink/trunk@156905 bbb929c8-8fbe-4397-9dbb-9b2b20218538