[PM] Add ability to safely downcast from Graph to GraphImpl.

This will allow us to use a single observer interface rather than having
public and private versions.

BUG=910288

Change-Id: Icd6ace3d79a03de28e947765edd65ec21618e85f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635637Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Auto-Submit: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664728}

chrome/browser/performance_manager/graph/graph_impl.cc

@@ -23,6 +23,13 @@ class UkmEntryBuilder;
 
 namespace performance_manager {
 
+namespace {
+
+// A unique type ID for this implementation.
+const uintptr_t kGraphImplType = reinterpret_cast<uintptr_t>(&kGraphImplType);
+
+}  // namespace
+
 GraphImpl::GraphImpl() {
   DETACH_FROM_SEQUENCE(sequence_checker_);
 }
@@ -44,6 +51,20 @@ GraphImpl::~GraphImpl() {
   DCHECK(nodes_.empty());
 }
 
+uintptr_t GraphImpl::GetImplType() const {
+  return kGraphImplType;
+}
+
+const void* GraphImpl::GetImpl() const {
+  return this;
+}
+
+// static
+GraphImpl* GraphImpl::FromGraph(const Graph* graph) {
+  CHECK_EQ(kGraphImplType, graph->GetImplType());
+  return reinterpret_cast<GraphImpl*>(const_cast<void*>(graph->GetImpl()));
+}
+
 void GraphImpl::RegisterObserver(GraphObserver* observer) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   observer->SetGraph(this);

chrome/browser/performance_manager/graph/graph_impl.h

@@ -50,6 +50,14 @@ class GraphImpl : public Graph {
   GraphImpl();
   ~GraphImpl() override;
 
+  // Graph implementation:
+  uintptr_t GetImplType() const override;
+  const void* GetImpl() const override;
+
+  // Helper function for safely downcasting to the implementation. This also
+  // casts away constness. This will CHECK on an invalid cast.
+  static GraphImpl* FromGraph(const Graph* graph);
+
   void set_ukm_recorder(ukm::UkmRecorder* ukm_recorder) {
     ukm_recorder_ = ukm_recorder;
   }

chrome/browser/performance_manager/graph/graph_impl_unittest.cc

@@ -15,6 +15,12 @@
 
 namespace performance_manager {
 
+TEST(GraphImplTest, SafeCasting) {
+  GraphImpl graph_impl;
+  const Graph* graph = &graph_impl;
+  EXPECT_EQ(&graph_impl, GraphImpl::FromGraph(graph));
+}
+
 TEST(GraphImplTest, FindOrCreateSystemNode) {
   GraphImpl graph;
 

chrome/browser/performance_manager/public/graph/graph.h

@@ -5,6 +5,8 @@
 #ifndef CHROME_BROWSER_PERFORMANCE_MANAGER_PUBLIC_GRAPH_GRAPH_H_
 #define CHROME_BROWSER_PERFORMANCE_MANAGER_PUBLIC_GRAPH_GRAPH_H_
 
+#include <cstdint>
+
 #include "base/macros.h"
 
 namespace performance_manager {
@@ -17,6 +19,12 @@ class Graph {
   Graph();
   virtual ~Graph();
 
+  // The following functions are implementation detail and should not need to be
+  // used by external clients. They provide the ability to safely downcast to
+  // the underlying implementation.
+  virtual uintptr_t GetImplType() const = 0;
+  virtual const void* GetImpl() const = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(Graph);
 };