Oilpan: clarify why FrameOwner is a GC mixin.

Add explanatory comment on why this interface derives from
GarbageCollectedMixin with Oilpan enabled.

R=
BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/841663002

git-svn-id: svn://svn.chromium.org/blink/trunk@187957 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed