Replace deprecated Callback with OnceCallback.

The original method declared the argument as OnceCallback:
https://cs.chromium.org/chromium/src/chrome/browser/offline_pages/background_loader_offliner.h?rcl=f1e0ecaf7d175cf0f4eb923daa10511be4b11931&l=75

This is part of the base::Callback migration.

Context: https://cs.chromium.org/chromium/src/docs/callback.md?rcl=9fcc3764aea8f97e9f6de4a9ee61d554e67edcda&l=40
Change-Id: I4f477760f4cf423d6352fd82041da75b2f0e1903
Bug: 714018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031405Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Auto-Submit: Jose Lopes <jabolopes@google.com>
Cr-Commit-Position: refs/heads/master@{#739106}

chrome/browser/offline_pages/background_loader_offliner_unittest.cc

@@ -234,9 +234,9 @@ class BackgroundLoaderOfflinerTest : public testing::Test {
     return base::BindOnce(&BackgroundLoaderOfflinerTest::OnCancel,
                           base::Unretained(this));
   }
-  base::Callback<void(bool)> const can_download_callback() {
-    return base::Bind(&BackgroundLoaderOfflinerTest::OnCanDownload,
-                      base::Unretained(this));
+  base::OnceCallback<void(bool)> can_download_callback() {
+    return base::BindOnce(&BackgroundLoaderOfflinerTest::OnCanDownload,
+                          base::Unretained(this));
   }
   Profile* profile() { return &profile_; }
   bool completion_callback_called() { return completion_callback_called_; }