Move PlatformEventDispatcher implementations to the Oilpan heap.

Source/core/frame/PlatformEventDispatcher.cpp

Index: Source/core/frame/PlatformEventDispatcher.cpp
diff --git a/Source/core/frame/PlatformEventDispatcher.cpp b/Source/core/frame/PlatformEventDispatcher.cpp
index 397d7881752d3dd435ddd6410d8a191368f72f71..5ed4d87af28dcbcb787c21c39ce6249fe219e0f9 100644
--- a/Source/core/frame/PlatformEventDispatcher.cpp
+++ b/Source/core/frame/PlatformEventDispatcher.cpp
@@ -16,10 +16,6 @@ PlatformEventDispatcher::PlatformEventDispatcher()
 {
 }
 
-PlatformEventDispatcher::~PlatformEventDispatcher()
-{
-}
-
 void PlatformEventDispatcher::addController(PlatformEventController* controller)
 {
     bool wasEmpty = m_controllers.isEmpty();
@@ -84,4 +80,21 @@ void PlatformEventDispatcher::notifyControllers()
         purgeControllers();
 }
 
+void PlatformEventDispatcher::trace(Visitor* visitor)
+{
+    visitor->trace(m_controllers);

[haraken, 2014/10/03 01:55:33]

Why do you trace m_controllers? If we trace m_controllers, it will strongly mark
all the controllers. Then clearWeakMembers will never run.

[sof, 2014/10/03 05:05:29]

It's a HeapVector<> of PlatformEventController* - that will mark the vector
backing store allocation, but not the elements. 

[haraken, 2014/10/03 05:36:01]

Makes sense. Shall we add a comment?

[sof, 2014/10/03 06:34:04]

Done. 

I incorrectly assumed that PlatformEventControllers were all on the heap by
default; they're not, m_controllers needs to be a WillBeHeapVector<> as a
result.

[haraken, 2014/10/03 10:10:56]

Can we move DeviceSingleWindowEventController, NavigatorGamepad and 
ScreenOrientationController to the heap at the same time?

It's confusing to use WillBeHeapVector because not all of
PlatformEventControllers are on the heap.

[sof, 2014/10/03 10:27:11]

NavigatorGamepad is a navigator supplement, so we cannot move it without also
having Navigator be on the heap always(*). Ditto for ScreenOrientationController
and LocalFrame.

* - assuming we don't want to reintroduce PersistentHeapSupplementables<>, which
I don't think we want to..?

+    visitor->registerWeakMembers<PlatformEventDispatcher, &PlatformEventDispatcher::clearWeakMembers>(this);
+}
+
+void PlatformEventDispatcher::clearWeakMembers(Visitor* visitor)
+{
+    for (size_t i = 0; i < m_controllers.size(); ++i) {
+        if (!visitor->isAlive(m_controllers[i])) {
+            m_controllers[i] = nullptr;
+            m_needsPurge = true;
+        }
+    }
+    // Next notification will purge the empty slots.
+}
+
 } // namespace blink