Adds a crash key to V8EventListener to help diagnose crbug.com/621730 .

chrome/common/crash_keys.cc

Index: chrome/common/crash_keys.cc
diff --git a/chrome/common/crash_keys.cc b/chrome/common/crash_keys.cc
index 225174cb82efdbe1c4b107b1c3b25f09b0a9a5d0..3b092a808222fda084f223b31d41cec93f92bb1e 100644
--- a/chrome/common/crash_keys.cc
+++ b/chrome/common/crash_keys.cc
@@ -194,6 +194,9 @@ size_t RegisterChromeCrashKeys() {
     // Temporary for https://crbug.com/612711.
     { "aci_wrong_sp_extension_id", kSmallSize },
 
+    // Temporary for http://crbug.com/621730
+    { "postmessage_script_info", kLargeSize },
+
     // Temporary for https://crbug.com/616149.
     { "existing_extension_pref_value_type", crash_keys::kSmallSize },
   };