Make ContentDecryptionModuleResult cross-thread destructible.

Source/platform/ContentDecryptionModuleResult.h

Index: Source/platform/ContentDecryptionModuleResult.h
diff --git a/Source/platform/ContentDecryptionModuleResult.h b/Source/platform/ContentDecryptionModuleResult.h
index a5a2b0f109ec4259bc804ef82da8dd1c98921ade..c480d776bb5623ec3ba374f10c8b52ddb57d8a6f 100644
--- a/Source/platform/ContentDecryptionModuleResult.h
+++ b/Source/platform/ContentDecryptionModuleResult.h
@@ -29,6 +29,11 @@ public:
         return WebContentDecryptionModuleResult(this);
     }
 
+    // WebContentDecryptionModuleResult may be destructed on a thread other
+    // than the Blink Oilpan thread that created it. To support that, mark
+    // it as cross-thread accessible (for WebContentDecryptionModuleResult's
+    // WebPrivatePtr<> to make use of.)
+    CROSS_THREAD_ACCESSIBLE();
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 };