GN: Make wtf_unittests build on mac/clang

On clang we enable -Wglobal-constructors which bans static initializers. This
is correct for all targets except unit test binaries, which need static initializers
since that's how the gtest TEST{,_F,_P} family of macros work. This splits that
warning into a separate config and avoids adding it to the unit test targets.

Review URL: https://codereview.chromium.org/318503002

git-svn-id: svn://svn.chromium.org/blink/trunk@175424 bbb929c8-8fbe-4397-9dbb-9b2b20218538