Change the way WebContentDecryptionModuleResult is passed.

Remove const-ref to make it clearer that the result is to be changed
by the caller.

BUG=358271

Review URL: https://codereview.chromium.org/343193006

git-svn-id: svn://svn.chromium.org/blink/trunk@176785 bbb929c8-8fbe-4397-9dbb-9b2b20218538