Add arg to change script for computing build timestamp

This makes it possible for other projects to closely
control the build timestamp without having to modify
compute_build_timestamp.py directly.

Bug: 944365
Change-Id: Ia79d9deae17b5bc47d19d6491caaadc8bb62657e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533319
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Eli Ribble <eliribble@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#644119}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: eecced2d8cc61596f2ffb28a06602d48bd39bed7

timestamp.gni

@@ -6,6 +6,17 @@
 
 import("//build/util/lastchange.gni")
 
+declare_args() {
+  # This should be the filename of a script that prints a single line
+  # containing an integer that's a unix timestamp in UTC.
+  # This timestamp is used as build time and will be compiled into
+  # other code.
+  #
+  # This argument may look unused. Before removing please check with the
+  # chromecast team to see if they still use it internally.
+  compute_build_timestamp = "compute_build_timestamp.py"
+}
+
 if (is_official_build) {
   official_name = "official"
 } else {
@@ -17,7 +28,7 @@ if (is_official_build) {
 # changes to keep this up to date.  (Bots run gn on each build, and for devs
 # the timestamp being 100% accurate doesn't matter.)
 # See compute_build_timestamp.py for tradeoffs for picking the timestamp.
-build_timestamp = exec_script("compute_build_timestamp.py",
+build_timestamp = exec_script(compute_build_timestamp,
                               [ official_name ],
                               "trim string",
                               [ lastchange_file ])