Reland of [tracing] Dump child processes' memory metrics in browser

base/process/process_metrics_linux.cc

 // 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.
 
 #include "base/process/process_metrics.h"
 
 #include <dirent.h>
 #include <fcntl.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 68 matching lines @@
         return 0;
       }
       size_t value;
       if (!StringToSizeT(split_value_str[0], &value)) {
         NOTREACHED();
         return 0;
       }
       return value;
     }
   }
-  NOTREACHED();
+  // This can be reached if the process dies when proc is read -- in that case,
+  // the kernel can return missing fields.
   return 0;
 }
 
 #if defined(OS_LINUX)
 // Read /proc/<pid>/sched and look for |field|. On succes, return true and
 // write the value for |field| into |result|.
 // Only works for fields in the form of "field    :     uint_value"
 bool ReadProcSchedAndGetFieldAsUint64(pid_t pid,
                                       const std::string& field,
                                       uint64_t* result) {
@@ skipping 841 matching lines @@
 #if defined(OS_LINUX)
 int ProcessMetrics::GetIdleWakeupsPerSecond() {
   uint64_t wake_ups;
   const char kWakeupStat[] = "se.statistics.nr_wakeups";
   return ReadProcSchedAndGetFieldAsUint64(process_, kWakeupStat, &wake_ups) ?
       CalculateIdleWakeupsPerSecond(wake_ups) : 0;
 }
 #endif  // defined(OS_LINUX)
 
 }  // namespace base