Fix the using of EnsurePtrConvertibleArg in a WebPrivateOwnPtr constructor

Before the change EnsurePtrConvertibleArg was used wrongly in WebPrivateOwnPtr(const PassOwnPtr<U>&) as it did not help to obviate ambiguous call problem.
Fixed now. A compile assertion is also added to the constructor in order to make sure that pointers to arrays are not converted (same as we have in OwnPtr constructor).

Review URL: https://codereview.chromium.org/27187002

git-svn-id: svn://svn.chromium.org/blink/trunk@159706 bbb929c8-8fbe-4397-9dbb-9b2b20218538