Avoid building blink_heap_unittests to unblock the Blink roll

The test is brand-new and is the only user of the code that's causing
link errors on GCC/component build.

BUG=332220
TBR=vegorov@chromium.org

Review URL: https://codereview.chromium.org/127063002

git-svn-id: svn://svn.chromium.org/blink/trunk@164638 bbb929c8-8fbe-4397-9dbb-9b2b20218538