Set crash key if WM_QUIT message is received on GPU message loop.

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 5af1a36a9ff50ec33e8abce9973ccd4fd2f46746..201c750933725f34342685257244173668038d3e 100644
--- a/blimp/engine/app/blimp_engine_crash_keys.cc
+++ b/blimp/engine/app/blimp_engine_crash_keys.cc
@@ -105,6 +105,9 @@ size_t RegisterEngineCrashKeys() {
       // Temporary for https://crbug.com/630496.
       { "swdh_get_registration_cannot_host_url", crash_keys::kLargeSize },
       { "swdh_get_registration_cannot_document_url", crash_keys::kLargeSize },
+
+      // Temporary for https://crbug.com/647068.
+      { "received_quit_message", crash_keys::kSmallSize },
     };
 
   return base::debug::InitCrashKeys(engine_keys, arraysize(engine_keys),