Hook up GN official build and branding.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index de1ee08b5bca140bf7fb6957b7f5b3685edf86f6..1110970a82acbbd660c41d25dff32bc30cf5214e 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -55,6 +55,16 @@ declare_args() {
   # Set to true on the command line when invoked by GYP. Build files can key
   # off of this to make any GYP-output-specific changes to the build.
   is_gyp = false
+
+  # Selects the desired build flavor. Official builds get additional
+  # processing to prepare for release. Normally you will want to develop and
+  # test with this flag off.
+  is_official_build = false
+
+  # Select the desired branding flavor. False means normal Chromium branding,
+  # true means official Google Chrome branding (requires extra Google-internal
+  # resources).
+  is_chrome_branded = false
 }
 
 # =============================================================================