The optimizing compilers actually invoke %TransitionElementsKind for
any kind of JSObject, the only relevant thing is the elements kind.
The runtime function was however checking for JSArray unnecessarily.
This only worked by coincindence in Crankshaft because the stub would
normally not call into the runtime fallback.

R=jarin@chromium.org
BUG=chromium:629823

Review-Url: https://codereview.chromium.org/2166963004
Cr-Commit-Position: refs/heads/master@{#37919}