Oilpan: fix build after r197063, move HitTestCache to the heap.

Source/core/layout/LayoutView.h

Index: Source/core/layout/LayoutView.h
diff --git a/Source/core/layout/LayoutView.h b/Source/core/layout/LayoutView.h
index c09b84d7628076b0b080b9281411d0ad8c2f4876..610f882a6625d05394cf31665c30fc5acab8388e 100644
--- a/Source/core/layout/LayoutView.h
+++ b/Source/core/layout/LayoutView.h
@@ -58,7 +58,7 @@ public:
     unsigned hitTestCount() const { return m_hitTestCount; }
     unsigned hitTestCacheHits() const { return m_hitTestCacheHits; }
 
-    void clearHitTestCache() { m_hitTestCache.clear(); }
+    void clearHitTestCache() { m_hitTestCache->clear(); }
 
     virtual const char* name() const override { return "LayoutView"; }
 
@@ -218,7 +218,7 @@ private:
 
     unsigned m_hitTestCount;
     unsigned m_hitTestCacheHits;
-    HitTestCache m_hitTestCache;
+    OwnPtrWillBePersistent<HitTestCache> m_hitTestCache;
 
     OwnPtrWillBePersistent<PendingSelection> m_pendingSelection;
 };