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/system_state.h b/system_state.h
index ce561e1..987cb6c 100644
--- a/system_state.h
+++ b/system_state.h
@@ -76,7 +76,7 @@
   virtual GpioHandler* gpio_handler() const = 0;
 
   // Returns a pointer to the update attempter object.
-  virtual UpdateAttempter* update_attempter() = 0;
+  virtual UpdateAttempter* update_attempter() const = 0;
 
   // Returns a pointer to the object that stores the parameters that are
   // common to all Omaha requests.