[SVG] Cleanup: remove externalResourcesRequired attribute from parser.

HTML spec update:
http://html5.org/tools/web-apps-tracker?from=8674&to=8675

BUG=332066

Review URL: https://codereview.chromium.org/329103003

git-svn-id: svn://svn.chromium.org/blink/trunk@176107 bbb929c8-8fbe-4397-9dbb-9b2b20218538