Triage failing w3c html template tests

The XHTML failing tests appear related to an already-filed bug (251183),
while the other tests marked "failure" are actually flaky (I've
fixed those to be marked flaky as well).

R=rafaelw@chromium.org
BUG=251183,330086

Review URL: https://codereview.chromium.org/166903004

git-svn-id: svn://svn.chromium.org/blink/trunk@167390 bbb929c8-8fbe-4397-9dbb-9b2b20218538