GN: Enable blink platform heap targets, put all tests behind flag

This puts the blink heap asm targets in the GN build set and enables
both blink platform test targets behind a single flag. They compile, link
and run successfully on mac but (probably) don't link yet on linux due
to ffmpeg deps.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/332123002

git-svn-id: svn://svn.chromium.org/blink/trunk@176174 bbb929c8-8fbe-4397-9dbb-9b2b20218538