Bug fix: Validate MLGraph's context in MLContext's compute()

This matches the Chromium prototype implementation.

Fixes webmachinelearning#341

index.bs

@@ -1040,6 +1040,7 @@ partial interface MLContext {
   <div class=algorithm-steps>
     1. Let |promise| be [=a new promise=].
     1. Return |promise| and run the following steps [=in parallel=]:
+        1. If |graph|.{{MLGraph/[[context]]}} is not [=this=], then [=reject=] |promise| with a "{{DataError}}" {{DOMException}}.
         1. If |graph|.{{MLGraph/[[context]]}}.{{MLContext/[[contextType]]}} is not "[=context type/default=]", [=reject=] |promise| with an "{{OperationError}}" {{DOMException}}.
         1. If [=validating graph resources=] given |inputs| and |graph|.{{MLGraph/[[inputDescriptors]]}} returns false, then [=reject=] |promise| with a "{{DataError}}" {{DOMException}}.
         1. If [=validating graph resources=] given |outputs| and |graph|.{{MLGraph/[[outputDescriptors]]}} returns false, then [=reject=] |promise| with a "{{DataError}}" {{DOMException}}.