Mac: fix PWA drag crash

content::GetContentClient()->browser() is null in the app shim, so we
currently crash when trying to fetch a default download file name.

A while ago, we discussed pumping the default name through Mojo, but
my thinking is that this is a lot of complexity for little gain.

This change uses a blank string as the default instead of trying to
fetch one from the browser if the browser is null. Empirically, in an
us/en locale on macOS 10.15.6, we end up with "download.png", which
is (IMO) better than the default "download" anyway. If this ends up
being a problem, we can always pipe through the default later, but
let's fix the crash for now.


Bug: 1019128
Change-Id: I5e941511b3c479841e172a30b72bf69c2c8782f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431546
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810634}

content/app_shim_remote_cocoa/web_drag_source_mac.mm

@@ -298,8 +298,11 @@ using content::DropData;
               &_downloadURL)) {
         // Generate the file name based on both mime type and proposed file
         // name.
-        std::string defaultName =
-            content::GetContentClient()->browser()->GetDefaultDownloadName();
+        std::string defaultName = content::GetContentClient()->browser()
+                                      ? content::GetContentClient()
+                                            ->browser()
+                                            ->GetDefaultDownloadName()
+                                      : std::string();
         mimeType = base::UTF16ToUTF8(mimeType16);
         _downloadFileName =
             net::GenerateFileName(_downloadURL,