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

chrome/browser/shell_integration_unittest.cc

Index: chrome/browser/shell_integration_unittest.cc
diff --git a/chrome/browser/shell_integration_unittest.cc b/chrome/browser/shell_integration_unittest.cc
index f17680f30387a111a67ca673dcf1b5266b2e21e8..9b95642b2b2b545684b07efa4c2d1fbaba578a2a 100644
--- a/chrome/browser/shell_integration_unittest.cc
+++ b/chrome/browser/shell_integration_unittest.cc
@@ -48,6 +48,10 @@ class MockEnvVarGetter : public base::EnvVarGetter {
     return false;
   }
 
+  virtual void SetEnv(const char* variable_name, const std::string& new_value) {
+    NOTIMPLEMENTED();
+  }
+
  private:
   std::map<std::string, std::string> variables_;