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/omaha_request_params.h b/omaha_request_params.h
index b04640c..b370bb7 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -247,6 +247,9 @@
   void set_current_channel(const std::string& channel) {
     current_channel_ = channel;
   }
+  void set_target_channel(const std::string& channel) {
+    target_channel_ = channel;
+  }
 
   // Enforce security mode for testing purposes.
   void SetLockDown(bool lock);