Use base::GetBuildTime in MetricsLog::GetBuildTime instead of reimplementing it.

components/metrics/metrics_log.cc

Index: components/metrics/metrics_log.cc
diff --git a/components/metrics/metrics_log.cc b/components/metrics/metrics_log.cc
index 28ed57b177cfbd19be50308286e7e76e01ac9943..8764a739fcd6af29cff51e3f6f6022fd16bcbe2b 100644
--- a/components/metrics/metrics_log.cc
+++ b/components/metrics/metrics_log.cc
@@ -10,6 +10,7 @@
 
 #include "base/base64.h"
 #include "base/basictypes.h"
+#include "base/build_time.h"
 #include "base/cpu.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
@@ -156,13 +157,8 @@ uint64 MetricsLog::Hash(const std::string& value) {
 // static
 int64 MetricsLog::GetBuildTime() {
   static int64 integral_build_time = 0;
-  if (!integral_build_time) {
-    base::Time time;
-    static const char kDateTime[] = __DATE__ " " __TIME__ " GMT";

[M-A Ruel, 2014/10/31 14:09:14]

Note that the PST / GMT difference was pure hack; base/build_time.cc totally
assumes PST, this totally assumes GMT, both are *wrong* but this one was more
wrong.

The right fix is:
- Remove this implementation (e.g. this CL is right).
- Fix base/build_time.cc to stop assuming PST (which is wrong, it's PDT most of
the time, it assumes timezone settings on the official builder) and use a
resource instead of __TIME__. Other solutions includes toolset specific things
like
http://stackoverflow.com/questions/5844701/how-do-i-get-the-utc-offset-for-time.
This is out of scope for this CL.

I recommend:
- filing a bug for the timezone issue with base::GetBuildTime()
- closing https://code.google.com/p/chromium/issues/detail?id=326158 as Fixed,
as this CL uses ToTimeT() which converts to UTC.
- Take the hit with metrics 1-day data inconsistency while people are upgraded
due to the bug here.

-    bool result = base::Time::FromString(kDateTime, &time);
-    DCHECK(result);
-    integral_build_time = static_cast<int64>(time.ToTimeT());
-  }
+  if (!integral_build_time)
+    integral_build_time = static_cast<int64>(base::GetBuildTime().ToTimeT());
   return integral_build_time;
 }