adding mimimum required of WebKit to build wtf

- based on commit: b44e402
  (https://chromium.googlesource.com/chromium/blink/+/b44e40209a29493f295537f120f5f2a82ebf3aa7)
adapting webkit config.gyp to reduce dependencies

- removing unit test target in order to avoid including gmock and gtest
- removing blink_gc_plugin option to avoid needing custom clang