Oilpan: Introduce WeakProcessingVisitor

Oilpan: Introduce WeakProcessingVisitor

The main purpose of this CL is to remove a use of s_markingVisitor,
since I'm going to remove the global variable in a follow-up CL
(in preparation for implementing a visitor for a snapshot GC).

This CL also add a bunch of ASSERTs to verify that a visitor for
weak processing does not do things that the visitor is not allowed to do.

BUG=490087
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196557

[haraken, 2015-06-05 04:19:48]

haraken@chromium.org changed reviewers:
+ keishi@chromium.org, oilpan-reviews@chromium.org

[haraken, 2015-06-05 04:19:49]

PTAL

[keishi, 2015-06-05 05:57:52]

lgtm

[haraken, 2015-06-05 05:58:18]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2015-06-05 05:58:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1168683002/1

[sof, 2015-06-05 06:00:04]

sigbjornf@opera.com changed reviewers:
+ sigbjornf@opera.com

[sof, 2015-06-05 06:00:04]

lgtm (i've kicked off some dbg bots to verify handling of ASSERT()s.)

[commit-bot: I haz the power, 2015-06-05 06:01:40]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196557