Add parsing for externalResourceRequired attribue in SVGGElement

There is a CL in progress to remove support for externalResourceRequired,
but in the meantime the ASSERT_NOT_REACHED hits in an Asan test case.
This adds parsing for it, which will be removed once I get around to
reviewing the other patch.

R=pdr@chromium.org,inferno@chromium.org
BUG=325282

Review URL: https://codereview.chromium.org/113023004

git-svn-id: svn://svn.chromium.org/blink/trunk@163976 bbb929c8-8fbe-4397-9dbb-9b2b20218538