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 09eac31a73cc3e6936b8bbe789477576736a835b..d858f50286808d84d5e46b60c9968601bf86ffe8 100644
--- a/blimp/engine/app/blimp_engine_crash_keys.cc
+++ b/blimp/engine/app/blimp_engine_crash_keys.cc
@@ -74,6 +74,9 @@ size_t RegisterEngineCrashKeys() {
       { "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 },
       { "postmessage_script_info", crash_keys::kLargeSize },
 
       // Temporary for https://crbug.com/616149.