Change ErrorCode into an enum class.

This change is needed in order for us to be able to import ErrorCode
symbols from chromeos_update_engine into chromeos_update_manager.
Unfortunately, shifting from plain 'enum' into an 'enum class' means
that the compiler treats the new class as a distinct type from int,
which in turn means that plenty of seamless arithmetic/bitwise
operations we used for manipulating error code values throughout the
code needed to be retrofitted with static_cast operators.

In the future, we should consider imposing a proper abstraction on
update engine error codes that'll prevent mingling with value encoding
directly and prevent such nastiness. It'll also make things more
coherent (types, semantics) and safer.

BUG=chromium:358329
TEST=Unit tests.

Change-Id: Ie55fa566b764cdab6c4785d995fb6daee4cb32d3
Reviewed-on: https://chromium-review.googlesource.com/203209
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/test_utils.h b/test_utils.h
index 43ff339..120f080 100644
--- a/test_utils.h
+++ b/test_utils.h
@@ -216,7 +216,7 @@
     if (this->HasOutputPipe()) {
       this->SetOutputObject(out_obj_);
     }
-    this->processor_->ActionComplete(this, kErrorCodeSuccess);
+    this->processor_->ActionComplete(this, ErrorCode::kSuccess);
   }
   static std::string StaticType() { return "ObjectFeederAction"; }
   std::string Type() const { return StaticType(); }
@@ -250,7 +250,7 @@
     if (this->HasInputObject()) {
       object_ = this->GetInputObject();
     }
-    this->processor_->ActionComplete(this, kErrorCodeSuccess);
+    this->processor_->ActionComplete(this, ErrorCode::kSuccess);
   }
   static std::string StaticType() { return "ObjectCollectorAction"; }
   std::string Type() const { return StaticType(); }