Move WebKitUnitTests.gyp out of Source/WebKit/chromium

We're close to removing Source/WebKit/chromium, which means we need to move all
the remaining files to new homes.

R=jamesr

Review URL: https://chromiumcodereview.appspot.com/21106004

git-svn-id: svn://svn.chromium.org/blink/trunk@155152 bbb929c8-8fbe-4397-9dbb-9b2b20218538