Oilpan: Fix isAlive dispatching for GarbageCollectedMixins.

Oilpan: Fix isAlive dispatching for GarbageCollectedMixins.

Update the test so that adjustment is needed and use a weak hash set
of mixin pointers.

I realized this didn't work when I needed to use it for the
Node hierarchy.

R=erik.corry@gmail.com, kouhei@chromium.org, oilpan-reviews@chromium.org, vegorov@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170688

[Mads Ager (chromium), 2014-04-02 18:13:55]

https://codereview.chromium.org/222653003/diff/20001/Source/heap/HeapTest.cpp
File Source/heap/HeapTest.cpp (right):

https://codereview.chromium.org/222653003/diff/20001/Source/heap/HeapTest.cpp...
Source/heap/HeapTest.cpp:178: virtual void virtualMethod() { }
I'll add a comment that this make sure that the object has a vtable which means
that adjustment becomes needed when a mixin is added in the inheritance
hierarchy.

[Erik Corry, 2014-04-02 18:21:13]

lgtm

[Mads Ager (chromium), 2014-04-02 18:30:50]

The CQ bit was checked by ager@chromium.org

[commit-bot: I haz the power, 2014-04-02 18:31:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ager@chromium.org/222653003/40001

[commit-bot: I haz the power, 2014-04-02 19:41:39]

Change committed as 170688

[haraken, 2014-04-03 04:01:10]

LGTM