Refactor libplatform unit tests to use GTest/GMock.

Also migrate them to src.

BUG=v8:3489
LOG=n
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/526043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23541 ce2b1a6d-e550-0410-aec6-3dcde31c8c00