[Sync] Implement MemoryDumpProvider.

base/trace_event/memory_usage_estimator.h

Index: base/trace_event/memory_usage_estimator.h
diff --git a/base/trace_event/memory_usage_estimator.h b/base/trace_event/memory_usage_estimator.h
index c089b0ee614d74910f849ac22fb9561a0027f42c..4f230c7b3890911c73c50d786a78514ac6ada83d 100644
--- a/base/trace_event/memory_usage_estimator.h
+++ b/base/trace_event/memory_usage_estimator.h
@@ -92,6 +92,11 @@ template <class T, class D>
 size_t EstimateMemoryUsage(const std::unique_ptr<T[], D>& array,
                            size_t array_length);
 
+// std::shared_ptr
+
+template <class T>
+size_t EstimateMemoryUsage(const std::shared_ptr<T>& ptr);
+
 // Containers
 
 template <class F, class S>
@@ -276,6 +281,28 @@ size_t EstimateMemoryUsage(const std::unique_ptr<T[], D>& array,
   return EstimateMemoryUsage(array.get(), array_length);
 }
 
+// std::shared_ptr
+
+template <class T>
+size_t EstimateMemoryUsage(const std::shared_ptr<T>& ptr) {
+  auto use_count = ptr.use_count();
+  if (use_count == 0) {
+    return 0;
+  }
+  // Model shared_ptr after libc++,
+  // see __shared_ptr_pointer from include/memory
+  struct SharedPointer {
+    void* vtbl;
+    long shared_owners;
+    long shared_weak_owners;
+    T* value;
+  };
+  // If object of size S shared N > S times we prefer to (potentially)
+  // overestimate than to return 0.
+  return sizeof(SharedPointer) +
+         (EstimateItemMemoryUsage(*ptr) + (use_count - 1)) / use_count;
+}
+
 // std::pair
 
 template <class F, class S>