Fix broken step reference in “fire a DND event” algorithm

A step in the “fire a DND event” algorithm referred back to an action occurring
in the “first step” of the algorithm, but since the time when that reference was
first made, additional steps were apparently added to the beginning of the
algorithm such that what had been the first step is now step 3. So this change
refines the algorithm to instead set and use an internal flag rather than
relying on a hardcoded reference back to a specific step.

Credit to @rniwa for catching it.

source

@@ -75709,6 +75709,8 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
 
   <ol>
 
+   <li>Let <var>dataDragStoreWasChanged</var> be false.</li>
+
    <li><p>If no specific <var>related target</var> was provided, set <var>related target</var> to
    null.</p></li>
 
@@ -75717,8 +75719,9 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
 
    <li>
 
-    <p>If <var>e</var> is <code data-x="event-dnd-dragstart">dragstart</code>, set the <span>drag
-    data store mode</span> to the <span data-x="concept-dnd-rw">read/write mode</span>.</p>
+    <p>If <var>e</var> is <code data-x="event-dnd-dragstart">dragstart</code>, then set the
+    <span>drag data store mode</span> to the <span data-x="concept-dnd-rw">read/write mode</span>
+    and set <var>dataDragStoreWasChanged</var> to true.</p>
 
     <p>If <var>e</var> is <code data-x="event-dnd-drop">drop</code>, set the <span>drag data store
     mode</span> to the <span data-x="concept-dnd-ro">read-only mode</span>.</p>
@@ -75850,8 +75853,8 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
    attribute. (It can only have changed value if <var>e</var> is <code
    data-x="event-dnd-dragstart">dragstart</code>.)</p></li>
 
-   <li><p>Set the <span>drag data store mode</span> back to the <span
-   data-x="concept-dnd-p">protected mode</span> if it was changed in the first step.</p></li>
+   <li><p>If <var>dataDragStoreWasChanged</var> is true, then set the <span>drag data store
+   mode</span> back to the <span data-x="concept-dnd-p">protected mode</span>.</p></li>
 
    <li><p>Break the association between <var>dataTransfer</var> and the <span>drag data
    store</span>.</p></li>