Make IDL Callbacks non-refcounted by default

Callbacks are different from other IDL-generated code, in that they invert
the flow of control of other interfaces (they let C++ code talk to JS, rather
than the other way around). For this reason, they don't interact with the GC,
and so can usually have a single owner rather than be managed with RefPtrs.

This patch converts most existing callbacks to be non-refcounted, and
instead manages them with OwnPtrs. Those callbacks where such conversion
was non-trivial are listed in IsLegacyRefCountedCallback() in
code_generator_v8.pm, and can be converted as follow-ons to this patch.

BUG=323681
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/85263006

git-svn-id: svn://svn.chromium.org/blink/trunk@162776 bbb929c8-8fbe-4397-9dbb-9b2b20218538