Make base::WeakPtr moveable.

base/memory/weak_ptr.cc

Index: base/memory/weak_ptr.cc
diff --git a/base/memory/weak_ptr.cc b/base/memory/weak_ptr.cc
index 16d3dff10aad842e3cd0ed53312e859cb739cd1a..4e77b04973cc944679d437cee831a433c0f88a97 100644
--- a/base/memory/weak_ptr.cc
+++ b/base/memory/weak_ptr.cc
@@ -34,14 +34,16 @@ WeakReference::Flag::~Flag() {
 WeakReference::WeakReference() {
 }
 
-WeakReference::WeakReference(const WeakReference& other) = default;
-
 WeakReference::WeakReference(const Flag* flag) : flag_(flag) {
 }
 
 WeakReference::~WeakReference() {
 }
 
+WeakReference::WeakReference(WeakReference&& other) = default;
+
+WeakReference::WeakReference(const WeakReference& other) = default;
+
 bool WeakReference::is_valid() const { return flag_.get() && flag_->IsValid(); }
 
 WeakReferenceOwner::WeakReferenceOwner() {