Revert of Oilpan: adjust GC policy under memory pressure.

Revert of Oilpan: adjust GC policy under memory pressure. (patchset #4 id:60001 of https://codereview.chromium.org/1174123002/)

Reason for revert:
Plausible candidate for regressing v8 GC totals on v8.top_25_smooth somewhat,

 https://code.google.com/p/chromium/issues/detail?id=499822

Original issue's description:
> Oilpan: adjust GC policy under memory pressure.
> 
> When notified of a v8 major GC, check if we're under memory pressure and
> overdue an Oilpan GC & urgently. If so, force a conservative GC right
> away.
> 
> This addresses dom-modify.html OOM troubles on win32, in particular. It
> runs into the allocation pattern where the Oilpan allocator is able to
> handle a tight loop just allocating Text nodes by bump allocating each
> such node. Each Text node has a large external string allocation however,
> which leads to a steep ramp-upin overall allocation amounts to the point
> where it might OOM as no Oilpan GCs will run. v8 will however notice the
> increasing memory pressure and schedule major GCs of its own.
> 
> Oilpan is notified after each v8 major GC, so use that opportunity to
> check for memory pressure & force an Oilpan GC right away.
> 
> R=haraken
> BUG=420515
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196953

TBR=haraken@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=420515
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197114

[sof, 2015-06-15 14:29:43]

Created Revert of Oilpan: adjust GC policy under memory pressure.

[commit-bot: I haz the power, 2015-06-15 14:30:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1191483002/1

[commit-bot: I haz the power, 2015-06-15 14:31:02]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197114

[haraken, 2015-06-15 15:28:24]

LGTM