base/process: Use /proc/self/stat to read startup time.

base/process/internal_linux.cc

 // Copyright (c) 2012 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/internal_linux.h"
 
 #include <limits.h>
 #include <unistd.h>
 
 #include <map>
@@ skipping 115 matching lines @@
 
 size_t GetProcStatsFieldAsSizeT(const std::vector<std::string>& proc_stats,
                                 ProcStatsFields field_num) {
   DCHECK_GE(field_num, VM_PPID);
   CHECK_LT(static_cast<size_t>(field_num), proc_stats.size());
 
   size_t value;
   return StringToSizeT(proc_stats[field_num], &value) ? value : 0;
 }
 
+int64_t ReadProcStatsAndGetFieldAsInt64(const std::string& pid,
+                                        ProcStatsFields field_num) {
+  std::string stats_data;
+  FilePath stat_file = FilePath(kProcDir).Append(pid).Append(kStatFile);
+  if (!ReadProcFile(stat_file, &stats_data))
+    return 0;
+  std::vector<std::string> proc_stats;

[jln (very slow on Chromium), 2017/02/22 22:37:15]

A little too much code duplication here. Perhaps refactor a tad if it's worth
it?  Up to you.

[Daniele Castagna, 2017/02/23 18:04:10]

Done.

+  if (!ParseProcStats(stats_data, &proc_stats))
+    return 0;
+  return GetProcStatsFieldAsInt64(proc_stats, field_num);
+}
+
 int64_t ReadProcStatsAndGetFieldAsInt64(pid_t pid, ProcStatsFields field_num) {
   std::string stats_data;
   if (!ReadProcStats(pid, &stats_data))
     return 0;
   std::vector<std::string> proc_stats;
   if (!ParseProcStats(stats_data, &proc_stats))
     return 0;
   return GetProcStatsFieldAsInt64(proc_stats, field_num);
 }
 
@@ skipping 60 matching lines @@
   // which means the answer is 100.
   // It may be the case that this value is always 100.
   static const int kHertz = sysconf(_SC_CLK_TCK);
 
   return TimeDelta::FromMicroseconds(
       Time::kMicrosecondsPerSecond * clock_ticks / kHertz);
 }
 
 }  // namespace internal
 }  // namespace base