Add UMA metrics for desktop memory pressure notifications.

base/process/process_metrics.h

 // Copyright (c) 2013 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.
 
 // This file contains routines for gathering resource statistics for processes
 // running on the system.
 
 #ifndef BASE_PROCESS_PROCESS_METRICS_H_
 #define BASE_PROCESS_PROCESS_METRICS_H_
 
@@ skipping 150 matching lines @@
   bool GetProportionalSetSizeBytes(uint64_t* pss_bytes) const;
 
 #if defined(OS_MACOSX)
   // Fills both CommitedKBytes and WorkingSetKBytes in a single operation. This
   // is more efficient on Mac OS X, as the two can be retrieved with a single
   // system call.
   bool GetCommittedAndWorkingSetKBytes(CommittedKBytes* usage,
                                        WorkingSetKBytes* ws_usage) const;
 
   struct TaskVMInfo {
-    // Only available on macOS 10.12+.
+    // Only available on macOS 10.12+. Will be 0 on previous versions.
     // Anonymous, non-discardable memory, including non-volatile IOKit.
     // Measured in bytes.
     uint64_t phys_footprint = 0;
 
     // Anonymous, non-discardable, non-compressed memory, excluding IOKit.
     // Measured in bytes.
     uint64_t internal = 0;
 
     // Compressed memory measured in bytes.
     uint64_t compressed = 0;
@@ skipping 350 matching lines @@
 // |size| and |info| are output parameters, only valid on Success.
 BASE_EXPORT MachVMRegionResult GetBasicInfo(mach_port_t task,
                                             mach_vm_size_t* size,
                                             mach_vm_address_t* address,
                                             vm_region_basic_info_64* info);
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
 }  // namespace base
 
 #endif  // BASE_PROCESS_PROCESS_METRICS_H_