Disable blink_platform_unittests and blink_heap_unittests in GN build

These targets do not link.

TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/352443002

git-svn-id: svn://svn.chromium.org/blink/trunk@176685 bbb929c8-8fbe-4397-9dbb-9b2b20218538