Prepare LayoutTests for making data: urls async

I've just about given up on making data urls async
for now, given the trouble I've had with browser_tests.
But I'm at least going to commit these changes
which prepare LayoutTests for making data urls async
at a future time.

BUG=308321, 365509

Review URL: https://codereview.chromium.org/258713015

git-svn-id: svn://svn.chromium.org/blink/trunk@172789 bbb929c8-8fbe-4397-9dbb-9b2b20218538