Reapply: [oilpan]: Change marking to do precise roots first and conservative

Source/platform/heap/HeapTest.cpp

Index: Source/platform/heap/HeapTest.cpp
diff --git a/Source/platform/heap/HeapTest.cpp b/Source/platform/heap/HeapTest.cpp
index da614ea97fa38df29465fc55727550954b0c2b52..63e794de907cab10110c842f7e45be98e6f11920 100644
--- a/Source/platform/heap/HeapTest.cpp
+++ b/Source/platform/heap/HeapTest.cpp
@@ -4672,8 +4672,6 @@ public:
             // shutting it down.
             stackPtrValue = reinterpret_cast<uintptr_t>(cto.get());
         }
-        RELEASE_ASSERT(stackPtrValue);
-
         // At this point it is "programatically" okay to shut down the worker thread
         // since the cto object should be dead. However out stackPtrValue will cause a
         // trace of the object when doing a conservative GC.
@@ -4697,6 +4695,13 @@ public:
         EXPECT_EQ(0, CrossThreadObject::s_destructorCalls);
         EXPECT_EQ(1, IntWrapper::s_destructorCalls);
 
+        // This release assert is here to ensure the stackValuePtr is not
+        // optimized away before doing the above conservative GC. If the
+        // EXPECT_EQ(0, CrossThreadObject::s_destructorCalls) call above
+        // starts failing it means we have to find a better way to ensure
+        // the stackPtrValue is not optimized away.
+        RELEASE_ASSERT(stackPtrValue);
+
         // Do a GC with no pointers on the stack to see the cto being collected.
         Heap::collectGarbage(ThreadState::NoHeapPointersOnStack);
         EXPECT_EQ(1, CrossThreadObject::s_destructorCalls);