Fix/extend unit test related functionality.

* Adds mock methods to UpdateAttempterMock and turning corresponding
  base methods into a virtual (needed for subsequent unit testing).

* Adds a setter to OmahaRequestParams.

* Limited general cleanup.

BUG=chromium:346914
TEST=Unit tests.

Change-Id: I0519ad5c43ddebabc1aff6585cf43a290a2081dc
Reviewed-on: https://chromium-review.googlesource.com/192660
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/real_system_state.h b/real_system_state.h
index ed24ed8..df1280f 100644
--- a/real_system_state.h
+++ b/real_system_state.h
@@ -69,7 +69,7 @@
     return gpio_handler_.get();
   }
 
-  virtual inline UpdateAttempter* update_attempter() {
+  virtual inline UpdateAttempter* update_attempter() const override {
     return update_attempter_.get();
   }