Convert some @font-face pixel tests to ref tests

BUG=231868,231870,231874

Review URL: https://chromiumcodereview.appspot.com/20020002

git-svn-id: svn://svn.chromium.org/blink/trunk@154955 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Share the Custom Element registration context with imported documents.

When a sub-document is imported with HTML Imports it should share and
contribute to the Custom Element definitions of the master document
through a shared registration context.

BUG=247219

Review URL: https://chromiumcodereview.appspot.com/20327002

git-svn-id: svn://svn.chromium.org/blink/trunk@154956 bbb929c8-8fbe-4397-9dbb-9b2b20218538