Linux: allow proxy PAC file configurations that use just a file path without a file:// prefix.

net/proxy/proxy_config_service_linux_unittest.cc

Index: net/proxy/proxy_config_service_linux_unittest.cc
===================================================================
--- net/proxy/proxy_config_service_linux_unittest.cc	(revision 142810)
+++ net/proxy/proxy_config_service_linux_unittest.cc	(working copy)
@@ -1098,6 +1098,21 @@
     },
 
     {
+      TEST_DESC("Valid PAC file without file://"),
+
+      // Input.
+      "[Proxy Settings]\nProxyType=2\n"
+          "Proxy Config Script=/wpad/wpad.dat\n",
+      {},                                      // env_values
+
+      // Expected result.
+      ProxyConfigService::CONFIG_VALID,
+      false,                         // auto_detect
+      GURL("file:///wpad/wpad.dat"),  // pac_url
+      ProxyRulesExpectation::Empty(),
+    },
+
+    {
       TEST_DESC("Per-scheme proxy rules"),
 
       // Input.