Makes GetBuildTime behave identically on all build types.

base/build_time.h

 // Copyright (c) 2011 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.
 
 #ifndef BASE_BUILD_TIME_H_
 #define BASE_BUILD_TIME_H_
 
 #include "base/base_export.h"
 #include "base/time/time.h"
 
 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 to midnight at the start of the day.

[agl, 2016/01/29 21:46:45]

s/rounded/rounded down/

(Normal rounding would suggest that it went to the nearest midnight, which might
be the future.)

s/ the day/ the day in UTC/ 

[M-A Ruel, 2016/01/30 02:58:14]

of the day in the current time zone, since __DATE__ is time local.

[Zachary Forman, 2016/02/01 07:28:30]

Done. I've adjusted it to use the result of a python script, so we have no
dependency on the time zone that we're currently building in :).

 //
-// 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 the __DATE__ macro, which doesn't trigger a rebuild when it
+// changes. However, official builds will always be built from scratch.
 //
-// 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 a day.

[agl, 2016/01/29 21:46:46]

s/a day/36 hours/

If you're just to the east of the dateline then you'll be the last in the world
to see the new day. Your build can then get stamped with the date (in your local
timezone) at midnight UTC, which is wrong by 36 hours.

[Zachary Forman, 2016/02/01 07:28:30]

Done. I've made it always use the real time at UTC, and hence narrowed this back
down to 24h.

 Time BASE_EXPORT GetBuildTime();
 
 }  // namespace base
 
 #endif  // BASE_BUILD_TIME_H_