Have VisitorHelper<> handle moving object registration.

As the registration of objects is only done by a compacting
GC, there isn't any need to be indirect about registering
them - remove a layer of Visitor virtual methods and have
the VisitorHelper<> do the registration directly.

R=haraken
BUG=681991

Review-Url: https://codereview.chromium.org/2642933005
Cr-Commit-Position: refs/heads/master@{#445245}
5 files changed