Reduce/remove usage of BrowserThread in content/browser/loader.

content/browser/loader/DEPS

 # The core loading code in this directory is being transitioned into a separate
 # Mojo networking service. See http://crbug.com/598073. The first stage is to
 # decouple the code that will be moved from the rest of content. These rules
 # help ensure that we don't add more includes, and can progressively reduce the
 # code's dependencies.
 #
 # For each file, the first section shows includes that are allowed (i.e. because
 # they are for files that will be moving as well). The second section, separated
 # by a comment, lists files that we will have to decouple.
 specific_include_rules = {
@@ skipping 306 matching lines @@
   ],
   "url_loader_factory_impl\.(cc|h)": [
     "-content",
     "+content/browser/loader/resource_dispatcher_host_impl.h",
     "+content/browser/loader/resource_requester_info.h",
     "+content/browser/loader/url_loader_factory_impl.h",
     "+content/common/content_export.h",
     "+content/common/resource_request.h",
     "+content/common/url_loader.mojom.h",
     "+content/common/url_loader_factory.mojom.h",
-    "+content/public/browser/browser_thread.h",
   ],
   "url_loader_factory_impl_unittest\.cc": [
     "-content",
     "+content/browser/child_process_security_policy_impl.h",
     "+content/browser/loader/mojo_async_resource_handler.h",
     "+content/browser/loader/test_url_loader_client.h",
     "+content/browser/loader/resource_dispatcher_host_impl.h",
     "+content/browser/loader/resource_message_filter.h",
     "+content/browser/loader/resource_request_info_impl.h",
     "+content/browser/loader/url_loader_factory_impl.h",
     "+content/browser/loader_delegate_impl.h",
     "+content/common/resource_request.h",
     "+content/common/resource_request_completion_status.h",
     "+content/common/url_loader.mojom.h",
     "+content/common/url_loader_factory.mojom.h",
     "+content/public/browser/resource_context.h",
     "+content/public/browser/resource_dispatcher_host_delegate.h",
     "+content/public/common/content_paths.h",
     "+content/public/test/test_browser_context.h",
     "+content/public/test/test_browser_thread_bundle.h",
 
     #TODO: To be removed when PlzNavigate lands.
     "+content/browser/loader/navigation_resource_throttle.h"
   ],
 }