Introduce MaybeHandle to police exception checking in handlified code.

src/v8globals.h

Index: src/v8globals.h
diff --git a/src/v8globals.h b/src/v8globals.h
index cb296146a7d59e13f88e4a9e3f63941a256a81df..8a89a933a7f08d7ffdfe905faa09fd1ec602542a 100644
--- a/src/v8globals.h
+++ b/src/v8globals.h
@@ -128,6 +128,7 @@ class MemoryChunk;
 class SeededNumberDictionary;
 class UnseededNumberDictionary;
 class NameDictionary;
+template <typename T> class MaybeHandle;
 template <typename T> class Handle;
 class Heap;
 class HeapObject;