FCMify two text-based-repaint tests

A previous patch (https://codereview.chromium.org/252353002) tried to
make all text-based-repaint tests use FCM, but caused flakiness reported
in http://crbug.com/367195 and http://crbug.com/367201.

It seems like the issue is that FCM wasn't being enabled soon enough. It
was turned on post-onload, after which some repaints may have been
generated. (This may no longer be true with repaint-after-layout.)

The fix is to enable FCM with the script include, rather than in
runRepaintTest.

Rather than causing churn on the entire tree, land an FCM version of
text-based-repaint.js and use it in two previous flaky tests. This
should indicate whether or not this was actually the problem.

BUG=368518

Review URL: https://codereview.chromium.org/267553002

git-svn-id: svn://svn.chromium.org/blink/trunk@173040 bbb929c8-8fbe-4397-9dbb-9b2b20218538