Do .data IDL conversion in constructor (#829)

* Do .data IDL conversion in PR constructor

* json parse, then convert

Author: marcoscaceres

index.html

@@ -663,6 +663,32 @@ <h2>
                     <var>paymentMethod</var>.<a data-lt="PaymentMethodData.data">data</a>
                     into a string. Rethrow any exceptions.
                   </li>
+                  <li>If <var>serializedData</var> is not null, and if required
+                  by the specification that defines the
+                  <var>paymentMethod</var>.<a>supportedMethods</a>:
+                    <ol>
+                      <li>Let <var>object</var> be the result of
+                        <a data-cite="ECMASCRIPT#sec-json.parse">JSON-parsing</a>
+                        <var>serializedData</var>.
+                      </li>
+                      <li>
+                        <p>
+                          <a data-lt="converting">Convert</a> <var>object</var>
+                          to an IDL value of the type specified by the
+                          specification that defines the
+                          <var>paymentMethod</var>.<a>supportedMethods</a>
+                          (e.g., a <a data-cite=
+                          "payment-method-basic-card#dom-basiccardrequest">BasicCardRequest</a>
+                          in the case of [[?payment-method-basic-card]]).
+                          Rethrow any exceptions.
+                        </p>
+                        <p class="note">
+                          This step assures that any IDL type conversion errors
+                          are caught as early as possible.
+                        </p>
+                      </li>
+                    </ol>
+                  </li>
                   <li>Add the tuple (<var>paymentMethod</var>.<a data-lt=
                   "PaymentMethodData.supportedMethods">supportedMethods</a>,
                   <var>serializedData</var>) to