Revert of [oilpan]: Change marking to do precise roots first and conservative second. (https://codereview.chromium.org/405403003/)

Reason for revert:
Broke blink_heap_unittests on http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/16773

Specifically, HeapTest.CrossThreadPointerToOrphanedPage fails with:

../../third_party/WebKit/Source/platform/heap/HeapTest.cpp:4685: Failure
Value of: CrossThreadObject::s_destructorCalls
  Actual: 1
Expected: 0

Original issue's description:
> [oilpan]: Change marking to do precise roots first and conservative second.
> 
> This way we avoid conservatively scanning objects that were reachable through precise pointers.
> 
> 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=178664

TBR=ager@chromium.org,erik.corry@gmail.com,haraken@chromium.org,oilpan-reviews@chromium.org,sigbjornf@opera.com,tkent@chromium.org,zerny@chromium.org,wibling@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/409183002

git-svn-id: svn://svn.chromium.org/blink/trunk@178671 bbb929c8-8fbe-4397-9dbb-9b2b20218538