Oilpan: move "forced GC" handling back to gcEpilogue().

Source/bindings/core/v8/V8GCController.cpp

Index: Source/bindings/core/v8/V8GCController.cpp
diff --git a/Source/bindings/core/v8/V8GCController.cpp b/Source/bindings/core/v8/V8GCController.cpp
index bf361334920664bf898ae1406214fc46c64510b5..ba589ea30307203035121e798cf4ac257028703a 100644
--- a/Source/bindings/core/v8/V8GCController.cpp
+++ b/Source/bindings/core/v8/V8GCController.cpp
@@ -394,9 +394,26 @@ void V8GCController::gcEpilogue(v8::GCType type, v8::GCCallbackFlags flags)
         minorGCEpilogue(isolate);
     } else if (type == v8::kGCTypeMarkSweepCompact) {
         majorGCEpilogue(isolate);
-        ThreadState::current()->didV8MajorGC(forceGC);
+        if (!forceGC)
+            ThreadState::current()->didV8MajorGC();
     }
 
+    if (forceGC) {
+        // This single GC is not enough for two reasons:
+        //   (1) The GC is not precise because the GC scans on-stack pointers conservatively.
+        //   (2) One GC is not enough to break a chain of persistent handles. It's possible that
+        //       some heap allocated objects own objects that contain persistent handles
+        //       pointing to other heap allocated objects. To break the chain, we need multiple GCs.
+        //
+        // Regarding (1), we force a precise GC at the end of the current event loop. So if you want
+        // to collect all garbage, you need to wait until the next event loop.
+        // Regarding (2), it would be OK in practice to trigger only one GC per gcEpilogue, because
+        // GCController.collectAll() forces 7 V8's GC.
+        Heap::collectGarbage(ThreadState::HeapPointersOnStack, ThreadState::GCWithSweep, Heap::ForcedGC);
+
+        // Forces a precise GC at the end of the current event loop.
+        ThreadState::current()->setGCState(ThreadState::FullGCScheduled);
+    }
     TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "UpdateCounters", TRACE_EVENT_SCOPE_THREAD, "data", InspectorUpdateCountersEvent::data());
 }