Move platform/mock files closer to their implementations

These are really layering violations in themselves.  These mocks
don't belong in the shipping Blink binary at all, but rather
should be implemented as part of testing/runner.

However changing that is out of scope for my refactoring.  This
at least makes platform/mock not depend on modules.

BUG=301947
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25225003

git-svn-id: svn://svn.chromium.org/blink/trunk@158549 bbb929c8-8fbe-4397-9dbb-9b2b20218538