Only set DropData::referrer_policy for 'downloadurl' mime type

The referrer policy is only used when dragging a link out of the WebView
results in a download. This is handled on Windows when the mime type is
'downloadurl'. This CL avoids storing the policy unconditionally when
converting the DragData Mojo type to DropData.

Bug: 1039255
Change-Id: Id0b94a7bddb274b53aba2e5c163407f0c875d95c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391068Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#804573}

content/renderer/drop_data_builder.cc

@@ -24,7 +24,6 @@ namespace content {
 // static
 DropData DropDataBuilder::Build(const WebDragData& drag_data) {
   DropData result;
-  result.referrer_policy = drag_data.ReferrerPolicy();
 
   for (const WebDragData::Item& item : drag_data.Items()) {
     switch (item.storage_type) {
@@ -41,6 +40,7 @@ DropData DropDataBuilder::Build(const WebDragData& drag_data) {
         }
         if (base::EqualsASCII(str_type, ui::kMimeTypeDownloadURL)) {
           result.download_metadata = item.string_data.Utf16();
+          result.referrer_policy = drag_data.ReferrerPolicy();
           break;
         }
         if (base::EqualsASCII(str_type, ui::kMimeTypeHTML)) {