base: Add SetEnv() to EnvVarGetter class and get rid of the some ifdefs.

net/proxy/proxy_config_service_linux_unittest.cc

Index: net/proxy/proxy_config_service_linux_unittest.cc
diff --git a/net/proxy/proxy_config_service_linux_unittest.cc b/net/proxy/proxy_config_service_linux_unittest.cc
index cf1c810a2db3b3224a8c0e51341e8d7fb2b3d1fb..501a411d05da6cb5a5891e6c9164517cf7b1353e 100644
--- a/net/proxy/proxy_config_service_linux_unittest.cc
+++ b/net/proxy/proxy_config_service_linux_unittest.cc
@@ -114,6 +114,9 @@ class MockEnvVarGetter : public base::EnvVarGetter {
     return false;
   }
 
+  virtual void SetEnv(const char* variable_name, const std::string& new_value) {
+    NOTIMPLEMENTED();
+  }
   // Intentionally public, for convenience when setting up a test.
   EnvVarValues values;