Refactoring: Get rid of isDocument() check in EventTarget.

This is a preparation for dispatching onload events
for sub-imports.

In a coming change, newly added executingWindow() will
walk up the import tree to return the master doc's DOMWindow.

TEST=none
BUG=240592
R=abarth,dglazkov

Review URL: https://chromiumcodereview.appspot.com/22564003

git-svn-id: svn://svn.chromium.org/blink/trunk@155921 bbb929c8-8fbe-4397-9dbb-9b2b20218538