Add StateOn{Moderate,Critical}NotificationReceived metrics

base/memory/memory_coordinator_client.h

Index: base/memory/memory_coordinator_client.h
diff --git a/base/memory/memory_coordinator_client.h b/base/memory/memory_coordinator_client.h
index 7d4681eed0ab270bfc8b9f8b98139bb28ae9db84..148f4c175345cd78af72506283ff76f04478231d 100644
--- a/base/memory/memory_coordinator_client.h
+++ b/base/memory/memory_coordinator_client.h
@@ -22,7 +22,9 @@ namespace base {
 // MemoryState is an indicator that processes can use to guide their memory
 // allocation policies. For example, a process that receives the suspended
 // state can use that as as signal to drop memory caches.
-enum class MemoryState {
+// NOTE: This enum is used to back an UMA histogram, and therefore should be
+// treated as append-only.
+enum class MemoryState : int {
   // The state is unknown.
   UNKNOWN = -1,
   // No memory constraints.
@@ -38,6 +40,8 @@ enum class MemoryState {
   SUSPENDED = 2,
 };
 
+const int kMemoryStateMax = static_cast<int>(MemoryState::SUSPENDED) + 1;
+
 // Returns a string representation of MemoryState.
 BASE_EXPORT const char* MemoryStateToString(MemoryState state);