base: Add UnSetEnv function to EnvVarGetter API.

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 c43e2072077099919a1c18940b95f2d9fe4ffaf3..30c33111ebe15f6678f4d74403ce8e01ed83ffa0 100644
--- a/net/proxy/proxy_config_service_linux_unittest.cc
+++ b/net/proxy/proxy_config_service_linux_unittest.cc
@@ -115,7 +115,12 @@ class MockEnvVarGetter : public base::EnvVarGetter {
   }
 
   virtual bool SetEnv(const char* variable_name, const std::string& new_value) {
-    NOTIMPLEMENTED();
+    ADD_FAILURE();
+    return false;
+  }
+
+  virtual bool UnSetEnv(const char* variable_name) {
+    ADD_FAILURE();
     return false;
   }