Make WeakContainer safe when weak_nsobject.h is included from ARC code.

base/ios/weak_nsobject.mm

Index: base/ios/weak_nsobject.mm
diff --git a/base/ios/weak_nsobject.mm b/base/ios/weak_nsobject.mm
index 36f9d3ea7cdd700da53d77a34245c3bad68a86c1..c017b1d18771ec8142c90082552c7b1352b2f2e8 100644
--- a/base/ios/weak_nsobject.mm
+++ b/base/ios/weak_nsobject.mm
@@ -12,6 +12,14 @@ namespace {
 char sentinelObserverKey_;
 }
 
+namespace base {
+
+WeakContainer::WeakContainer(id object) : object_(object) {}
+
+WeakContainer::~WeakContainer() {}
+
+}  // namespace base
+
 @interface CRBWeakNSProtocolSentinel ()
 // Container to notify on dealloc.
 @property(readonly, assign) scoped_refptr<base::WeakContainer> container;