AU: Detect and report public key verify failures, but continue updating.

Make a new OmahaEvent (Type: 14 (kTypeUpdateDownloadFinished), Result:
0 (kResultError), ErrorCode: 18
(kActionCodeDownloadPayloadPubKeyVerificationError)). This event is
sent if we have a successful download that fails public key
verification. Currently this is a non-fatal failure, but we plan to
make it fatal in future clients.

BUG=chromium-os:13341
TEST=unittests; 3 on device tests: w/o public key on device, w/ pub
key and matching download, w/ pub key and non-matching download

Change-Id: Ib6589c78449c2dfcbffa4c85ab679f7fe844584b

Review URL: http://codereview.chromium.org/6792065
diff --git a/action_processor.h b/action_processor.h
index ef06181..7f7862a 100644
--- a/action_processor.h
+++ b/action_processor.h
@@ -41,6 +41,7 @@
   kActionCodeNewRootfsVerificationError = 15,
   kActionCodeNewKernelVerificationError = 16,
   kActionCodeSignedDeltaPayloadExpectedError = 17,
+  kActionCodeDownloadPayloadPubKeyVerificationError = 18,
   kActionCodeOmahaRequestEmptyResponseError = 200,
   kActionCodeOmahaRequestXMLParseError = 201,
   kActionCodeOmahaRequestNoUpdateCheckNode = 202,