GN: add is_official_build

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f14a027af5b090771d11880cf27b4ea7fd640818..05fe3500ffdaa5335346ef07fc57f55144008bfe 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -85,6 +85,10 @@ config("skia_private") {
       "SK_GAMMA_CONTRAST=0.0",
     ]
   }
+  if (is_official_build || is_android) {
+    # TODO(bsalomon): it'd be nice to make Android normal.
+    defines += [ "SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=0" ]
+  }
 }
 
 # Any code that's linked into Skia-the-library should use this config via += skia_library_configs.