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..8014cf1d242815b93dddddecc1c791186de16cdb 100644
--- a/base/build_time.h
+++ b/base/build_time.h
@@ -10,17 +10,13 @@
 
 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. However, official builds will always be built from scratch.

[M-A Ruel, 2016/02/04 15:24:13]

Technically, this is not 100% true. Adding Anthony so he can clarify this.

[laforge, 2016/02/04 15:50:19]

That's true for our full release builds, however for our continuous official
builds those do not get clobbered.

[Zachary Forman, 2016/02/04 22:39:26]

Do you have any suggestion as to how you'd like continuous official builds to be
handled?

 //
-// 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.
 Time BASE_EXPORT GetBuildTime();
 
 }  // namespace base