Add an action to create a symbol file for the Blimp engine

This is modeled after what is done for the official Chrome linux build.
As it stands right now, the breakpad symbol file is roughly 300MB and
takes a couple of minutes to run, so the action should only be triggered
if this is an official release.

BUG=597454

Review-Url: https://codereview.chromium.org/1979773002
Cr-Original-Commit-Position: refs/heads/master@{#394191}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 060ef497fef78501475548ec61dbc8c290f02a9b

args/bots/official.desktop/blimp-engine.gn

@@ -3,5 +3,8 @@ import("//build/args/blimp_engine.gn")
 # Use release build to improve engine performance, especially WebP encoding.
 is_debug = false
 
+# Designate this as an official build to pick up breakpad symbols.
+is_official_build = true
+
 # Turn on DCHECK to find bugs.
 dcheck_always_on = true