Allow redundant calls to claimInterface() and releaseInterface()

This change updates the steps for claimInterface() and releaseInterface()
so that calls requesting the state that the device is already in cause
the Promise returned to be resolved without any further action taken.

Fixes WICG#161.

index.bs

@@ -955,42 +955,45 @@ parallel:
      and <a>resolve</a> |promise|.
 
 The {{USBDevice/claimInterface(interfaceNumber)}} method, when invoked, MUST
-return a new {{Promise}} and run the following steps <a>in parallel</a>:
+return a new {{Promise}} |promise| and run the following steps <a>in
+parallel</a>:
 
   1. If the device is no longer connected to the system, <a>reject</a> |promise|
      with a {{NotFoundError}} and abort these steps.
-  2. Let |interface| be the interface in the <a>active configuration</a> with
+  1. Let |interface| be the interface in the <a>active configuration</a> with
      <code>bInterfaceNumber</code> equal to |interfaceNumber|. If no such
      interface exists, <a>reject</a> |promise| with a {{NotFoundError}} and
      abort these steps.
-  3. If <code>|device|.{{USBDevice/opened}}</code> is not <code>true</code> or
-     <code>|interface|.{{USBInterface/claimed}}</code> is not <code>false</code>,
+  1. If <code>|device|.{{USBDevice/opened}}</code> is not <code>true</code>,
      <a>reject</a> |promise| with an {{InvalidStateError}} and abort these
      steps.
-  4. Perform the necessary platform-specific steps to request exclusive control
+  1. If <code>|interface|.{{USBInterface/claimed}}</code> is <code>true</code>,
+     <a>resolve</a> |promise| and abort these steps.
+  1. Perform the necessary platform-specific steps to request exclusive control
      over |interface| for the current execution context. If this fails,
      <a>reject</a> |promise| with a {{NetworkError}} and abort these steps.
-  5. Set <code>|interface|.{{USBInterface/claimed}}</code> to <code>true</code>
+  1. Set <code>|interface|.{{USBInterface/claimed}}</code> to <code>true</code>
      and <a>resolve</a> |promise|.
 
 The {{USBDevice/releaseInterface(interfaceNumber)}} method, when invoked, MUST
-return a new {{Promise}} |promise| and run the following steps in
-parallel:
+return a new {{Promise}} |promise| and run the following steps <a>in
+parallel</a>:
 
   1. Let |device| be the target {{USBDevice}} object.
-  2. If |device| is no longer connected to the system, <a>reject</a> |promise|
+  1. If |device| is no longer connected to the system, <a>reject</a> |promise|
      with a {{NotFoundError}} and abort these steps.
-  3. Let |interface| be the interface in the <a>active configuration</a> with
+  1. Let |interface| be the interface in the <a>active configuration</a> with
      <code>bInterfaceNumber</code> equal to |interfaceNumber|. If no such
      interface exists, <a>reject</a> |promise| with a {{NotFoundError}} and
      abort these steps.
-  4. If <code>|device|.{{USBDevice/opened}}</code> or
-     <code>|interface|.{{USBInterface/claimed}}</code> is not <code>true</code>,
+  1. If <code>|device|.{{USBDevice/opened}}</code> is not <code>true</code>,
      <a>reject</a> |promise| with an {{InvalidStateError}} and abort these
      steps.
-  5. Perform the necessary platform-specific steps to reliquish exclusive
+  1. If <code>|interface|.{{USBInterface/claimed}}</code> is <code>false</code>,
+     <a>resolve</a> |promise| and abort these steps.
+  1. Perform the necessary platform-specific steps to reliquish exclusive
      control over |interface|.
-  6. Set <code>|interface|.{{USBInterface/claimed}}</code> to <code>false</code>
+  1. Set <code>|interface|.{{USBInterface/claimed}}</code> to <code>false</code>
      and <a>resolve</a> |promise|.
 
 The {{USBDevice/selectAlternateInterface(interfaceNumber, alternateSetting)}}