Set [[extractable]] and [[usages]] in the deriveKey method (#384)

These parameters were accidentally not set on the returned object.
The new text matches that in `importKey`, and the expected behavior
in the web platform tests.

Author: panva

spec/Overview.html

@@ -1983,6 +1983,19 @@ <h4>The deriveKey method</h4>
                   |usages| is empty, then [= exception/throw =] a {{SyntaxError}}.
                 </p>
               </li>
+              <li>
+                <p>
+                  Set the {{CryptoKey/[[extractable]]}} internal
+                  slot of |result| to |extractable|.
+                </p>
+              </li>
+              <li>
+                <p>
+                  Set the {{CryptoKey/[[usages]]}} internal
+                  slot of |result| to the [= normalized
+                    value =] of |usages|.
+                </p>
+              </li>
               <li>
                 <p>
                   Resolve |promise| with