Makes GetBuildTime behave identically on all build types.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 946d17f3de33bde297556cba53be41b4952e4b89..80a3981ea7e2566106e6bfe1120e88d3e10de8f1 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -24,6 +24,12 @@ import("//build/config/ui.gni")
 import("//build/nocompile.gni")
 import("//testing/test.gni")
 
+declare_args() {
+  # Override this value to give a specific build date.
+  # See //base/build_time.cc for more details.
+  override_build_date = "N/A"

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

What about:

- By default override_build_date is set to the first of the month. This means
normal builds will be deterministic for the month only. This means that on the
first of each month, the cache will be invalidated, causing significant load. :/
- On official build, override_build_date defaults to "N/A".

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

I like this - we could actually use a python script, and actually use 'real'
UTC, as well.

+}
+
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
@@ -1317,6 +1323,10 @@ component("base") {
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 
+  if (override_build_date != "N/A") {
+    defines += [ "BUILD_TIME=\"" + override_build_date + "\"" ]
+  }
+
   allow_circular_includes_from = public_deps
 }