Adds a crash key to V8EventListener to help diagnose crbug.com/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 db8460bb758cc5cf71dbd46e913f14d8dad0331d..09eac31a73cc3e6936b8bbe789477576736a835b 100644
--- a/blimp/engine/app/blimp_engine_crash_keys.cc
+++ b/blimp/engine/app/blimp_engine_crash_keys.cc
@@ -73,6 +73,9 @@ size_t RegisterEngineCrashKeys() {
       // Temporary for https://crbug.com/612711.
       { "aci_wrong_sp_extension_id", crash_keys::kSmallSize },
 
+      // Temporary for http://crbug.com/621730.
+      { "postmessage_script_info", crash_keys::kLargeSize },
+
       // Temporary for https://crbug.com/616149.
       { "existing_extension_pref_value_type", crash_keys::kSmallSize },
     };