Add StateOn{Moderate,Critical}NotificationReceived metrics

base/memory/memory_coordinator_client.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MEMORY_MEMORY_COORDINATOR_CLIENT_H_
 #define BASE_MEMORY_MEMORY_COORDINATOR_CLIENT_H_
 
 #include "base/base_export.h"
 
 namespace base {
 
 // OVERVIEW:
 //
 // MemoryCoordinatorClient is an interface which a component can implement to
 // respond to memory state changes. Unlike MemoryPressureListener, this is a
 // stateful mechanism and clients receive notifications only when memory states
 // are changed. State transitions are throttled to avoid thrashing; the exact
 // throttling period is platform dependent, but will be at least 5-10 seconds.
 // Clients are expected to make changes in memory usage that persist for the
 // duration of the memory state.
 
 // 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.
   NORMAL = 0,
   // Running and interactive but allocation should be throttled.
   // Clients should free up any memory that is used as an optimization but
   // that is not necessary for the process to run (e.g. caches).
   THROTTLED = 1,
   // Still resident in memory but core processing logic has been suspended.
   // Clients should free up any memory that is used as an optimization, or
   // any memory whose contents can be reproduced when transitioning out of
   // the suspended state (e.g. parsed resource that can be reloaded from disk).
   SUSPENDED = 2,
 };
 
+const int kMemoryStateMax = static_cast<int>(MemoryState::SUSPENDED) + 1;
+
 // Returns a string representation of MemoryState.
 BASE_EXPORT const char* MemoryStateToString(MemoryState state);
 
 // This is an interface for components which can respond to memory status
 // changes. An initial state is NORMAL. See MemoryCoordinatorClientRegistry for
 // threading guarantees and ownership management.
 class BASE_EXPORT MemoryCoordinatorClient {
  public:
   // Called when memory state has changed. Any transition can occur except for
   // UNKNOWN. General guidelines are:
   //  * NORMAL:    Restore the default settings for memory allocation/usage if
   //               it has changed.
   //  * THROTTLED: Use smaller limits for memory allocations and caches.
   //  * SUSPENDED: Purge memory.
   virtual void OnMemoryStateChange(MemoryState state) = 0;
 
 protected:
   virtual ~MemoryCoordinatorClient() {}
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_COORDINATOR_CLIENT_H_