extensions::APIRequestHandler::StartRequest signature change.

No longer needs `thread` param.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/16395195c71cc831e85e10524f2c18311b325df5

commit 16395195c71cc831e85e10524f2c18311b325df5
Author: Clark DuVall <cduvall@chromium.org>
Date:   Tue Jul 30 18:04:24 2019 +0000

    Remove unused IOThreadExtensionFunction

    This also merges UIThreadExtensionFunction and ExtensionFunction since
    there's no need for them to be separate anymore. This was made possible
    because WebRequest has moved to the UI thread, and it was the only API
    using IOThreadExtensionFunction.

    Follow up CL will remove references to UIThreadExtensionFunction.

    Bug: 980774

extensions/renderer/brave_shields_content_setting.cc

@@ -157,8 +157,7 @@ void BraveShieldsContentSetting::HandleFunction(const std::string& method_name,
   parse_result.arguments->Insert(0u, std::make_unique<base::Value>(pref_name_));
   request_handler_->StartRequest(
       context, "braveShields." + method_name, std::move(parse_result.arguments),
-      parse_result.callback, v8::Local<v8::Function>(),
-      binding::RequestThread::UI);
+      parse_result.callback, v8::Local<v8::Function>());
 }
 
 }  // namespace extensions