Allow inline event handlers in imported documents.

This change allows inline script running, if the document
has "context" frame, that is the frame of the master document.

TEST=import-events-inline.html
BUG=334773

Review URL: https://codereview.chromium.org/130123003

git-svn-id: svn://svn.chromium.org/blink/trunk@165441 bbb929c8-8fbe-4397-9dbb-9b2b20218538