Expose limits on the adapter (gpuweb#489)

There's no reason to leave this out anymore.
Implementations can implement it whenever they want.

spec/index.bs

@@ -328,7 +328,7 @@ and exposes its capabilities (extensions and limits).
 interface GPUAdapter {
     readonly attribute DOMString name;
     readonly attribute object extensions;
-    //readonly attribute GPULimits limits; Don't expose higher limits for now.
+    readonly attribute object limits;
 
     // May reject with DOMException  // TODO: DOMException("OperationError")?
     Promise<GPUDevice> requestDevice(optional GPUDeviceDescriptor descriptor = {});
@@ -354,6 +354,11 @@ interface GPUAdapter {
             not by the [=adapter=], it will be `false`.
           - If an extension is supported by the user agent and
             by the [=adapter=], it will be `true`.
+
+    : <dfn>limits</dfn>
+    ::
+        A {{GPULimits}} object which exposes the maximum limits supported
+        by the adapter.
 </dl>
 
 {{GPUAdapter}} also has the following internal slots: