GN: Add all_blink target

This is the GN equivalent of public/all.gyp's all_blink target which
defines all test targets that blink contains. As more test targets start
linking in the GN build they should be added to this file. For now, this
just includes wtf_unittests and the /platform/ and /core/ components.

Review URL: https://codereview.chromium.org/324963007

git-svn-id: svn://svn.chromium.org/blink/trunk@175917 bbb929c8-8fbe-4397-9dbb-9b2b20218538