Use broadcasting definition in expand(), rather than bespoke steps

This matches the Chromium implementation.

index.bs

@@ -2842,17 +2842,12 @@ partial interface MLGraphBuilder {
     <div class="note">
         The permissions and context validity have been checked by [[#api-mlgraphbuilder-constructor]] steps.
     </div>
-    1. If any of the following steps fail, then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
-        1. Let |inputDesc| be |input|.{{MLOperand/[[descriptor]]}}.
-        1. If the sequence length of |newShape| is not equal to the [=rank=] of |inputDesc|, then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
-        1. Let |outputDesc| be a copy of |inputDesc|.
-        1. [=list/For each=] |index| in [=the range=] 0 to the [=rank=] of |input|, exclusive:
-            Issue: Can this be replaced with [=unidirectionally broadcasting the shapes=] |inputDesc| and |newShape|.
-            1. Let |size| be the |input|.{{MLOperand/shape()}}[|index|].
-            1. If |size| is not equal to 1 and not equal to |newShape|[index], then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
-            1. If |size| is equal to 1, then let |outputDesc|.{{MLOperandDescriptor/dimensions}}[|index|] be |newShape|[|index|].
+    1. Let |descriptor| be a new {{MLOperandDescriptor}}.
+    1. Set |descriptor|.{{MLOperandDescriptor/dataType}} to |input|.{{MLOperand/[[descriptor]]}}.{{MLOperandDescriptor/dataType}}.
+    1. Set |descriptor|.{{MLOperandDescriptor/dimensions}} to the result of [=unidirectionally broadcasting the shapes=] |input|.{{MLOperand/[[descriptor]]}}.{{MLOperandDescriptor/dimensions}} and |newShape|.
+        1. If that returns failure, then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
     1. If any of the following sub-steps fail, [=exception/throw=] an "{{OperationError}}" {{DOMException}}.
-        1. Let |output| be the result of [=creating an MLOperand=] given [=this=] and |outputDesc|.
+        1. Let |output| be the result of [=creating an MLOperand=] given [=this=] and |descriptor|.
         1. Make a request to the underlying platform to:
             1. Create [=platform operator=] |expandImpl| for this method, given |input| and |newShape|.
             1. Set |output|.{{MLOperand/[[operator]]}} to |expandImpl|.