IDL compiler: sync Python to r166999

      Python side to:
Add cross-origin BindingsSecurity checks to 'EventTarget::dispatchEvent'.
https://codereview.chromium.org/150203016

Adds test case too.

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/166593002

git-svn-id: svn://svn.chromium.org/blink/trunk@167179 bbb929c8-8fbe-4397-9dbb-9b2b20218538