Fix crash in WebLayer if the permission prompt for storage access is interrupted.

Bug: 1123212
Change-Id: I8d5d3a23d9d8c7902d37cf19faa3a1dbfe898ed8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399574
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805168}

weblayer/browser/java/org/chromium/weblayer_private/DownloadCallbackProxy.java

@@ -65,7 +65,7 @@ public final class DownloadCallbackProxy {
 
         String[] requestPermissions = new String[] {permission.WRITE_EXTERNAL_STORAGE};
         window.requestPermissions(requestPermissions, (permissions, grantResults) -> {
-            if (grantResults[0] == PackageManager.PERMISSION_DENIED) {
+            if (grantResults.length == 0 || grantResults[0] == PackageManager.PERMISSION_DENIED) {
                 DownloadCallbackProxyJni.get().allowDownload(callbackId, false);
                 return;
             }