Oilpan: fix Windows build after r178431.

With http://crrev.com/406523002 making GarbageCollectedMixin a non-abstract
class, MSVC elicits a warning that DLL exported GCMixin classes have a
base class that isn't DLL exported.

Add the needed annotation to GarbageCollectedMixin to address (as warnings
are considered as errors.)

R=haraken@chromium.org,wibling@chromium.org
BUG=382557
NOTRY=true

Review URL: https://codereview.chromium.org/396153003

git-svn-id: svn://svn.chromium.org/blink/trunk@178439 bbb929c8-8fbe-4397-9dbb-9b2b20218538