Add ErrorCode::kUserCanceled.

To accomodate the posibility of canceling an ongoing update, this patch
adds a new ErrorCode signaling a user canceling the update.

This patch also removes the now useless unittests that checks if you
added a new error code since the compiler fails if you don't add the
error code to the method mentioned in the test.

Bug: None
TEST=FEATURES=test emerge-link update_engine
TEST=`mmma system/update_engine` on edison-eng and aosp_arm-eng

Change-Id: I991dab1dd8b6c0bb0c24d51cca4e1b75440a86b4
diff --git a/metrics.h b/metrics.h
index ec9d880..bb9fa5e 100644
--- a/metrics.h
+++ b/metrics.h
@@ -149,6 +149,7 @@
   kVerificationFailed,          // Root or Kernel partition verification failed.
   kPostInstallFailed,           // The postinstall step failed.
   kAbnormalTermination,         // The attempt ended abnormally.
+  kUpdateCanceled,              // Update canceled by the user.
 
   kNumConstants,