Remove support for the externalResourcesRequired attribute.

Intent: https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/intent$20to$20remove/blink-dev/cyBqba-FxQU/fL208UQx3sEJ

Changes in behavior:

1. Load events should only be dispatched to elements that have some actual external content to load and to the outermost svg element.
2. Scripts should be killed/not executed if the svg is not xml wellformed.
3. Load events on elements that have external resources to fetch will not be dispatched until that resource is ready for use, similar to how externalResourcesRequired="true" used to work.

BUG=332066

Review URL: https://codereview.chromium.org/62083002

git-svn-id: svn://svn.chromium.org/blink/trunk@164692 bbb929c8-8fbe-4397-9dbb-9b2b20218538