Improve the error message in BuildTime.InThePast to help figure out the problem.

base/build_time.cc

Index: base/build_time.cc
diff --git a/base/build_time.cc b/base/build_time.cc
index 6f496f2efd2a9c482151911d8ecad02873935ce3..cedc8a5eef9e641c94513fbff96a1c26b1c6fc91 100644
--- a/base/build_time.cc
+++ b/base/build_time.cc
@@ -15,6 +15,7 @@ namespace base {
 Time GetBuildTime() {
   Time integral_build_time;
   // BUILD_DATE is exactly "Mmm DD YYYY".
+  // See //build/write_build_date_header.py.
   const char kDateTime[] = BUILD_DATE " 05:00:00";
   bool result = Time::FromUTCString(kDateTime, &integral_build_time);
   DCHECK(result);