Make HistoryBackend a client of memory coordinator

components/memory_coordinator/browser/memory_coordinator.cc

Index: components/memory_coordinator/browser/memory_coordinator.cc
diff --git a/components/memory_coordinator/browser/memory_coordinator.cc b/components/memory_coordinator/browser/memory_coordinator.cc
index 9291fd00f9a042270315839ea7146d3ae899a6fa..c1119c98e4bdc57e27ecc5e14778cfab9f405619 100644
--- a/components/memory_coordinator/browser/memory_coordinator.cc
+++ b/components/memory_coordinator/browser/memory_coordinator.cc
@@ -4,6 +4,8 @@
 
 #include "components/memory_coordinator/browser/memory_coordinator.h"
 
+#include "components/memory_coordinator/common/memory_coordinator_features.h"
+
 namespace memory_coordinator {
 
 // The implementation of MemoryCoordinatorHandle. See memory_coordinator.mojom
@@ -31,6 +33,14 @@ class MemoryCoordinatorHandleImpl : public mojom::MemoryCoordinatorHandle {
   DISALLOW_COPY_AND_ASSIGN(MemoryCoordinatorHandleImpl);
 };
 
+// static
+MemoryCoordinator* MemoryCoordinator::GetInstance() {
+  if (!IsEnabled())
+    return nullptr;
+  return base::Singleton<MemoryCoordinator,
+                         base::LeakySingletonTraits<MemoryCoordinator>>::get();
+}
+
 MemoryCoordinator::MemoryCoordinator()
     : pressure_listener_(
           base::Bind(&MemoryCoordinator::OnMemoryPressure,