Add a pure virtual method to the GarbageCollectedMixin class

It's easy to miss adding USING_GARBAGE_COLLECTED_MIXIN macros to classes
that inherit from GarbageCollectedMixin. To avoid the programming error,
this CL adds a pure virtual method to the GarbageCollectedMixin class.
If we miss the USING_GARBAGE_COLLECTED_MIXIN macro, we'll hit a compile error.

This CL also fixes existing programming errors.

BUG=420515

Review URL: https://codereview.chromium.org/659953002

git-svn-id: svn://svn.chromium.org/blink/trunk@183799 bbb929c8-8fbe-4397-9dbb-9b2b20218538