Add a static getter for ChildMemoryCoordinator

Currently ChildMemoryCoordinator is owned by RenderThreadImpl but
some other components (like cc) would want to become clients of
ChildMemoryCoordinator. Add a static getter to provide access
to ChildMemoryCoordinator.

BUG=617492

Review-Url: https://codereview.chromium.org/2291473002
Cr-Commit-Position: refs/heads/master@{#415212}

components/memory_coordinator/child/child_memory_coordinator_impl.cc

@@ -4,17 +4,39 @@
 
 #include "components/memory_coordinator/child/child_memory_coordinator_impl.h"
 
+#include "base/lazy_instance.h"
+#include "base/synchronization/lock.h"
+
 namespace memory_coordinator {
 
+namespace {
+
+base::LazyInstance<base::Lock>::Leaky g_lock = LAZY_INSTANCE_INITIALIZER;
+ChildMemoryCoordinatorImpl* g_child_memory_coordinator = nullptr;
+
+}  // namespace
+
+// static
+ChildMemoryCoordinatorImpl* ChildMemoryCoordinatorImpl::GetInstance() {
+  base::AutoLock lock(*g_lock.Pointer());
+  return g_child_memory_coordinator;
+}
+
 ChildMemoryCoordinatorImpl::ChildMemoryCoordinatorImpl(
     mojom::MemoryCoordinatorHandlePtr parent,
     ChildMemoryCoordinatorDelegate* delegate)
     : binding_(this), parent_(std::move(parent)), delegate_(delegate) {
+  base::AutoLock lock(*g_lock.Pointer());
   DCHECK(delegate_);
+  DCHECK(!g_child_memory_coordinator);
+  g_child_memory_coordinator = this;
   parent_->AddChild(binding_.CreateInterfacePtrAndBind());
 }
 
 ChildMemoryCoordinatorImpl::~ChildMemoryCoordinatorImpl() {
+  base::AutoLock lock(*g_lock.Pointer());
+  DCHECK(g_child_memory_coordinator == this);
+  g_child_memory_coordinator = nullptr;
 }
 
 void ChildMemoryCoordinatorImpl::OnStateChange(mojom::MemoryState state) {

components/memory_coordinator/child/child_memory_coordinator_impl.h

@@ -31,8 +31,11 @@ class MEMORY_COORDINATOR_EXPORT ChildMemoryCoordinatorImpl
     : public ClientRegistry,
       NON_EXPORTED_BASE(public mojom::ChildMemoryCoordinator) {
  public:
-  explicit ChildMemoryCoordinatorImpl(mojom::MemoryCoordinatorHandlePtr parent,
-                                      ChildMemoryCoordinatorDelegate* delegate);
+  // Returns the instance of ChildMemoryCoordinatorImpl. Could be nullptr.
+  static ChildMemoryCoordinatorImpl* GetInstance();
+
+  ChildMemoryCoordinatorImpl(mojom::MemoryCoordinatorHandlePtr parent,
+                             ChildMemoryCoordinatorDelegate* delegate);
   ~ChildMemoryCoordinatorImpl() override;
 
   // mojom::ChildMemoryCoordinator implementations: