Resource requests from Save-Page-As should go through CanRequestURL checks.

      This CL:

- Added checks to ResourceDispatcherHostImpl::BeginSaveFile to verify if
  the renderer process is authorized to access a given resource.

- Removed separate code path for file: URIs that used to be implemented
  in SaveFileManager::SaveLocalFile.  Avoiding a separate code path
  helps consolidate all authorization checks in one place.

BUG=616429

Review-Url: https://codereview.chromium.org/2075273002
Cr-Commit-Position: refs/heads/master@{#408235}