Check if storage permission is granted before requesting it

In DownloadController.requestFileAccess, we should check if storage
permission is already granted before requesting it.

Bug: 836397
Change-Id: Idadcdfc3b055696b8f7194e5d63bdee54a8d4959
Reviewed-on: https://chromium-review.googlesource.com/1029116Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554579}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadController.java

@@ -175,6 +175,11 @@ public class DownloadController {
             return;
         }
 
+        if (delegate.hasPermission(permission.WRITE_EXTERNAL_STORAGE)) {
+            callback.onResult(Pair.create(true, null));
+            return;
+        }
+
         if (!delegate.canRequestPermission(permission.WRITE_EXTERNAL_STORAGE)) {
             callback.onResult(Pair.create(false,
                     delegate.isPermissionRevokedByPolicy(permission.WRITE_EXTERNAL_STORAGE)