Pepper: Fix 2 message rewriting issues in NaClMessageScanner.
1) Don't stop rewriting if a SerializedVar contains no handles. It's
   possible a subsequent one could.
2) Stop rewriting if a resource reply message is found to contain no
   handles. The message will be discarded at the end.

This was split out from the HandleConverter CL to make it more comprehensible.

tbr=dmichael@chromium.org

BUG=194304

Review URL: https://codereview.chromium.org/56923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232621 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232621