Remove the dependence on BrowserThread::FILE for

android_webview/browser/aw_browser_context.cc

Index: android_webview/browser/aw_browser_context.cc
diff --git a/android_webview/browser/aw_browser_context.cc b/android_webview/browser/aw_browser_context.cc
index 22d70ed6161f08e5e4e1294ccbe353c8f2d32275..d4a3b10636b5a758346c07c2902b9dfdb462a155 100644
--- a/android_webview/browser/aw_browser_context.cc
+++ b/android_webview/browser/aw_browser_context.cc
@@ -78,8 +78,7 @@ AwBrowserContext* g_browser_context = NULL;
 std::unique_ptr<net::ProxyConfigService> CreateProxyConfigService() {
   std::unique_ptr<net::ProxyConfigService> config_service =
       net::ProxyService::CreateSystemProxyConfigService(
-          BrowserThread::GetTaskRunnerForThread(BrowserThread::IO),
-          nullptr /* Ignored on Android */);
+          BrowserThread::GetTaskRunnerForThread(BrowserThread::IO));
 
   // TODO(csharrison) Architect the wrapper better so we don't need a cast for
   // android ProxyConfigServices.