Use Platform::current()->createThread instead of WTF::createThread in heap unit tests.

Removes the final usages of WTF::createThread so we can remove it.

R=jochen@chromium.org, oilpan-reviews@chromium.org, wibling@chromium.org
BUG=416376

Review URL: https://codereview.chromium.org/589653003

git-svn-id: svn://svn.chromium.org/blink/trunk@182435 bbb929c8-8fbe-4397-9dbb-9b2b20218538