Move finishJSTest to fire at onload.

This fixes a bunch of cases of having TEST COMPLETE get printed
after shouldBe lines.

-Make the compositing/gestures/gesture-tapHighlight reftests
not needlessly import js-test.js.
-Fix some webgl tests to set jsTestIsAsync.
-Change successfullyParsed to always be a boolean instead of undefined.
-Some tests never fire an onload event, so they now don't print TEST COMPLETE.

Review URL: https://codereview.chromium.org/59793005

git-svn-id: svn://svn.chromium.org/blink/trunk@161493 bbb929c8-8fbe-4397-9dbb-9b2b20218538