Add crash keys to LocalDOMWindow to help diagnose bug 621730.

blimp/engine/app/blimp_engine_crash_keys.cc

Index: blimp/engine/app/blimp_engine_crash_keys.cc
diff --git a/blimp/engine/app/blimp_engine_crash_keys.cc b/blimp/engine/app/blimp_engine_crash_keys.cc
index 50149fbe6891825bb63c8eedd7c7a58da5f626b4..084355c76a893d1cb5f49998151468ced607ad49 100644
--- a/blimp/engine/app/blimp_engine_crash_keys.cc
+++ b/blimp/engine/app/blimp_engine_crash_keys.cc
@@ -69,6 +69,11 @@ size_t RegisterEngineCrashKeys() {
       // Temporary for https://crbug.com/612711.
       { "aci_wrong_sp_extension_id", crash_keys::kSmallSize },
 
+      // Temporary for http://crbug.com/621730.
+      { "postmessage_src_origin", crash_keys::kMediumSize },
+      { "postmessage_dst_origin", crash_keys::kMediumSize },
+      { "postmessage_dst_url", crash_keys::kLargeSize },
+
       // Temporary for https://crbug.com/616149.
       { "existing_extension_pref_value_type", crash_keys::kSmallSize },
     };