Cleanup: Simplify platform unittests targets.

It is seems redundant to have a separate target only for the main()
entry.

It looks cleaner to have everything contained into a single target imo.

BUG=None
TEST=blink_heap_unittests,blink_platform_unittests
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/430933005

git-svn-id: svn://svn.chromium.org/blink/trunk@179353 bbb929c8-8fbe-4397-9dbb-9b2b20218538