update_engine: Report VPD write failure to UMA

Currently we see many first actives to come from non-FSI images. But we have not
been able to figure out why. This CL, reports a new error
kFirstActiveOmahaPingSentPersistenceError when writing the first active omaha
flag into VPD fails. This allows us to see if that is the actual cause of the
problem.

CL:1062659 adds the enum value on the Chrome side.

BUG=chromium:833980
TEST=unittests
TEST=precq

Change-Id: I65e233c5f895489ba905494fb20d7b00d0c4af10
Reviewed-on: https://chromium-review.googlesource.com/1062662
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/common/fake_hardware.h b/common/fake_hardware.h
index d699fb7..d68b0f8 100644
--- a/common/fake_hardware.h
+++ b/common/fake_hardware.h
@@ -110,8 +110,9 @@
     return first_active_omaha_ping_sent_;
   }
 
-  void SetFirstActiveOmahaPingSent() override {
+  bool SetFirstActiveOmahaPingSent() override {
     first_active_omaha_ping_sent_ = true;
+    return true;
   }
 
   // Setters