Makes GetBuildTime behave identically on all build types.

base/build_time.h

Index: base/build_time.h
diff --git a/base/build_time.h b/base/build_time.h
index 4f0abc381336641425dab76d4f1533a0041b22bb..87a9d2eb3886faae0b67323c448bdd83c44405f3 100644
--- a/base/build_time.h
+++ b/base/build_time.h
@@ -10,17 +10,15 @@
 
 namespace base {
 
-// GetBuildTime returns the time at which the current binary was built.
+// GetBuildTime returns the time at which the current binary was built,
+// rounded down to midnight at the start of the day in UTC.
 //
-// This uses the __DATE__ and __TIME__ macros, which don't trigger a rebuild
-// when they change. However, official builds will always be rebuilt from
-// scratch.
+// This uses a generated file, which doesn't trigger a rebuild when the time
+// changes. It will, however, be updated whenever //build/util/LASTCHANGED
+// changes.
 //
-// Also, since __TIME__ doesn't include a timezone, this value should only be
-// considered accurate to a day.
-//
-// NOTE: This function is disabled except for the official builds, by default
-// the date returned is "Sep 02 2008 08:00:00 PST".
+// This value should only be considered accurate to within a day.
+// It will always be in the past.

[M-A Ruel, 2016/02/09 00:14:13]

It is guaranteed to be in the past in UTC.

[Zachary Forman, 2016/02/09 00:47:18]

The time is given with respect to UTC. If you take a look at base/time.h, it
indicates that all times are considered relative to the windows epoch:

// Time represents an absolute point in coordinated universal time (UTC),
// internally represented as microseconds (s/1,000,000) since the Windows epoch
// (1601-01-01 00:00:00 UTC). System-dependent clock interface routines are
// defined in time_PLATFORM.cc.

Regardless of what timezone you're in, Time::Now() will be converted into an
internal time - and that will always be /after/ the time that chromium was
built.

 Time BASE_EXPORT GetBuildTime();
 
 }  // namespace base