Add cursor.request (#257)

Define new |request| attribute on IDBCursor. Discussed in #255

WPT PR ready to land: web-platform-tests/wpt#15820

Implementations pending, but this should be fairly straightforward so going ahead with landing this (and tests) optimistically.

index.bs

@@ -4617,6 +4617,7 @@ interface IDBCursor {
   readonly attribute IDBCursorDirection direction;
   readonly attribute any key;
   readonly attribute any primaryKey;
+  readonly attribute IDBRequest request;
 
   void advance([EnforceRange] unsigned long count);
   void continue(optional any key);
@@ -4657,12 +4658,16 @@ enum IDBCursorDirection {
       Returns the [=cursor/effective key=] of the cursor.
       Throws a "{{InvalidStateError}}" {{DOMException}} if the cursor is advancing or is finished.
     </dd>
+    <dt><var>cursor</var> . {{IDBCursor/request}}
+    <dd>
+      Returns the [=cursor/request=] that was used to obtain this cursor.
+    </dd>
   </dl>
 </div>
 
 
 The <dfn attribute for=IDBCursor>source</dfn> attribute's getter must
-return the [=cursor/source=] of this [=cursor=]. This
+return the [=cursor/source=] of the [=cursor=]. This
 attribute never returns null or throws an exception, even if the
 cursor is currently being iterated, has iterated past its end, or its
 [=/transaction=] is not [=transaction/active=].
@@ -4690,6 +4695,15 @@ object is modified, those modifications will be seen by anyone
 inspecting the value of the cursor. However modifying such an object
 does not modify the contents of the database.
 
+The <dfn attribute for=IDBCursor>request</dfn> attribute's getter must
+return the [=cursor/request=] of the [=cursor=].
+
+<aside class=advisement>
+  &#x1F6A7;
+  The {{IDBCursor/request}} attribute is new in this edition.
+  &#x1F6A7;
+</aside>
+
 <div class=note>
   The following methods advance a [=cursor=]. Once the cursor has
   advanced, a <a event>`success`</a> event will be fired at the
@@ -7184,6 +7198,7 @@ For the revision history of the second edition, see [that document's Revision Hi
 * Updated [partial interface definition](#global-scope) since {{WindowOrWorkerGlobalScope}} is now a `mixin` ([PR #238](https://github.com/w3c/IndexedDB/pull/238)).
 * Added {{IDBFactory/databases()}} method. ([Issue #31](https://github.com/w3c/IndexedDB/issues/31))
 * Added {{IDBTransaction/commit()}} method. ([Issue #234](https://github.com/w3c/IndexedDB/issues/234))
+* Added {{IDBCursor/request}} attribute. ([Issue #255](https://github.com/w3c/IndexedDB/issues/255))
 
 <!-- ============================================================ -->
 # Acknowledgements # {#acknowledgements}
@@ -7204,6 +7219,7 @@ for his general authoring advice.
 
 Special thanks to
 Chris Anderson,
+Jake Archibald,
 Andreas Butler,
 Pablo Castro,
 Victor Costan,