Revert of Revert "Revert of [oilpan]: Change marking to do precise roots first and conservative

Revert of Revert "Revert of [oilpan]: Change marking to do precise roots first and conservative second. (http… (patchset #2 of https://codereview.chromium.org/413133006/)

Reason for revert:
This breaks strongification of ephemerons when there are pointers on the stack. See:

https://codereview.chromium.org/461413002/

for details.


Original issue's description:
> Revert "Revert of [oilpan]: Change marking to do precise roots first and conservative second. (https://codereview.chromium.org/405403003/)"
> 
> This reverts commit 765af3a45b15c4e5f65810c8ed8e20d44da04034.
> 
> AFAICT the CrossThreadPointerToOrphanedPage test was a bit flaky in the sense that the compiler was free to reuse the slot where the stackPtrValue was located after the RELEASE_ASSERT. However the test assumed the stackPtrValue was on the stack (to mimic a rogue integer value finding a dead object) when doing the conservative GC further down.
> After my change to Heap::CollectGarbage it seemed the android compiler no longer kept the value around and hence the test started failing. I have now moved the RELEASE_ASSERT down to after the point where we do the conservative GC which makes the test pass on android again.
> 
> I uploaded the unfixed diff first and then a second diff (moved RELEASE_ASSERT) with the fix.
> 
> R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, sigbjornf@opera.com, tkent@chromium.org, zerny@chromium.org
> BUG=
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178845

NOTRY=true
NOTREECHECKS=true

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180160

[Mads Ager (chromium), 2014-08-13 09:07:54]

Created Revert of Revert "Revert of [oilpan]: Change marking to do precise roots
first and conservative second. (http…

[commit-bot: I haz the power, 2014-08-13 09:08:56]

The CQ bit was unchecked by commit-bot@chromium.org

[haraken, 2014-08-13 09:11:05]

LGTM

[Mads Ager (google), 2014-08-13 09:16:11]

The CQ bit was checked by ager@google.com

[commit-bot: I haz the power, 2014-08-13 09:18:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ager@chromium.org/464253002/1

[Mads Ager (chromium), 2014-08-13 09:21:44]

The CQ bit was unchecked by ager@chromium.org

[Mads Ager (chromium), 2014-08-13 09:21:52]

The CQ bit was checked by ager@chromium.org

[commit-bot: I haz the power, 2014-08-13 09:22:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ager@chromium.org/464253002/1

[commit-bot: I haz the power, 2014-08-13 09:23:22]

Change committed as 180160

[Mads Ager (chromium), 2014-08-13 11:20:36]

A revert of this CL (patchset #1) has been created in
https://codereview.chromium.org/468083002/ by ager@chromium.org.

The reason for reverting is: This change did not contain the bug it just made it
much more likely that we would see it. Reapplying (again). :).