Bug 1398471 - Read data from DataTransfer before yielding, r=gijs

Previously the code looped and yielded between each loop. This caused the
DataTransfer to be cleared before the read completed.

This splits the loop into 2 sections such that we read all important data from
the DataTransfer before it is cleared.

browser/components/places/content/controller.js

@@ -1598,6 +1598,10 @@ var PlacesControllerDragHelper = {
     duplicable.set(PlacesUtils.TYPE_UNICODE, new Set());
     duplicable.set(PlacesUtils.TYPE_X_MOZ_URL, new Set());
 
+    // Collect all data from the DataTransfer before processing it, as the
+    // DataTransfer is only valid during the synchronous handling of the `drop`
+    // event handler callback.
+    let dtItems = [];
     for (let i = 0; i < dropCount; ++i) {
       let flavor = this.getFirstValidFlavor(dt.mozTypesAt(i));
       if (!flavor)
@@ -1610,7 +1614,10 @@ var PlacesControllerDragHelper = {
           continue;
         handled.add(data);
       }
+      dtItems.push({flavor, data});
+    }
 
+    for (let {flavor, data} of dtItems) {
       let nodes;
       if (flavor != TAB_DROP_TYPE) {
         nodes = PlacesUtils.unwrapNodes(data, flavor);