macOS Signing Scripts: Create the notarization sentinel file.

The internal signing scripts will look for the presence of the sentinel
(notarize_me.txt) as an indicator that the build should be submitted for
notarization.

Bug: 850199
Change-Id: I1c67a5c891b9d099510b1deaebce90378d134ca4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679056Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672583}

chrome/installer/mac/BUILD.gn

@@ -11,6 +11,7 @@ import("//chrome/process_version_rc_template.gni")
 group("mac") {
   public_deps = [
     ":copies",
+    ":notarization_sentinel",
   ]
 }
 
@@ -62,6 +63,24 @@ copy("copy_signing") {
   ]
 }
 
+action("notarization_sentinel") {
+  visibility = [ ":mac" ]
+
+  script = "//build/redirect_stdout.py"
+
+  _output = "$_packaging_dir/notarize_me.txt"
+
+  args = [
+    rebase_path(_output, root_out_dir),
+    "echo",
+    "yes",
+  ]
+
+  outputs = [
+    _output,
+  ]
+}
+
 copy("copies") {
   visibility = [ ":mac" ]