Revert of Oilpan: Implement a GC to take a heap snapshot

Revert of Oilpan: Implement a GC to take a heap snapshot (patchset #7 id:120001 of https://codereview.chromium.org/1159773004/)

Reason for revert:
ThreadedWeakness unit test is failing on various bots,

 http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%2032/builds/36384
 http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/32383

Original issue's description:
> Oilpan: Implement a GC to take a heap snapshot
> 
> This CL implements Heap::collectGarbage(gcType = TakeSnapshot), which is intended
> to be used to take a heap snapshot when onMemoryDump is requested etc.
> 
> The GC is expected to be as side-effect-free as possible. Thus the GC behaves as follows:
> 
> 1) Run a marking task.
> 2) Take a heap snapshot (The actual implementation will come in a follow-up CL).
> 3) Clear all marks on marked objects. No sweeping task runs.
> 
> More contexts:
> https://codereview.chromium.org/1149673002/
> https://groups.google.com/a/chromium.org/d/topic/oilpan-reviews/6LhByzmVt6Q/discussion
> 
> BUG=490087
> TEST=HeapTest.ThreadedWeakness
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195982
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196059

TBR=oilpan-reviews@chromium.org,keishi@chromium.org,primiano@chromium.org,ssid@chromium.org,haraken@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=490087
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196063

[sof, 2015-05-28 14:11:15]

Created Revert of Oilpan: Implement a GC to take a heap snapshot

[commit-bot: I haz the power, 2015-05-28 14:11:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143243006/1

[commit-bot: I haz the power, 2015-05-28 14:11:55]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196063

[haraken, 2015-05-28 14:21:07]

LGTM, thanks for the revert. Will investigate.