Add Permissions Policy Integration and initial createContext() hooks

Fix #145

index.bs

@@ -474,13 +474,40 @@ interface ML {
 };
 </script>
 
+The {{ML/createContext()}} method steps are:
+1. If the [=responsible document=] is not [=allowed to use=] the [=webnn-feature|webnn=] feature, then throw a "{{SecurityError!!exception}}" {{DOMException}} and abort these steps.
+1. Let |context| be a new {{MLContext}} object.
+1. Switch on the method's first argument:
+    <dl class=switch>
+    <dt>{{MLContextOptions}}
+    <dd>Set |context|'s [=context type=] to [=default-context|default=].
+
+    <dt>{{WebGLRenderingContext}}
+    <dd>Set |context|'s [=context type=] to [=webgl-context|webgl=].
+
+    <dt>{{GPUDevice}}
+    <dd>Set |context|'s [=context type=] to [=webgpu-context|webgpu=].
+
+    <dt>Otherwise
+    <dd>Set |context|'s [=context type=] to [=default-context|default=].
+    </dl>
+1. Return |context|.
+
+### Permissions Policy Integration ### {#permissions-policy-integration}
+
+This specification defines a <a>policy-controlled feature</a> identified by the
+string "<code><dfn data-lt="webnn-feature">webnn</dfn></code>".
+Its <a>default allowlist</a> is <code>'self'</code>.
+
 ## MLContext ## {#api-mlcontext}
 The {{MLContext}} interface represents a global state of neural network compute workload and execution processes.
 <script type=idl>
 [SecureContext, Exposed=Window]
 interface MLContext {};
 </script>
 
+The <dfn>context type</dfn> for an {{MLContext}} is either "<code><dfn data-lt="default-context">default</dfn></code>", "<code><dfn data-lt="webgl-context">webgl</dfn></code>" or "<code><dfn data-lt="webgpu-context">webgpu</dfn></code>".
+
 ## MLOperandDescriptor ## {#api-mloperanddescriptor}
 <script type=idl>
 enum MLInputOperandLayout {