Add a check for callback in RunCallbackOnIOThread.

This should catch the cases when the callback is unexpectedly null and
provide a more useful call stack.

I'm not convinced this will trigger because the code already
de-references callback 2 lines later but I don't have a better lead at
the moment.

Bug: 889972
Change-Id: If63da6f5eaa611e9369da7c70cfcfc784695f01c
Reviewed-on: https://chromium-review.googlesource.com/c/1323721Reviewed-by: Nathan Parker <nparker@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606359}

components/safe_browsing/android/safe_browsing_api_handler_bridge.cc

@@ -36,6 +36,7 @@ void RunCallbackOnIOThread(
     std::unique_ptr<SafeBrowsingApiHandler::URLCheckCallbackMeta> callback,
     SBThreatType threat_type,
     const ThreatMetadata& metadata) {
+  CHECK(callback);  // Remove after fixing https://crbug.com/889972
   base::PostTaskWithTraits(
       FROM_HERE, {BrowserThread::IO},
       base::BindOnce(std::move(*callback), threat_type, metadata));