Dictionary prototypes don't have stable maps, but still don't matter for
element access. Generalized the JSNativeContextSpecialization a bit to
handle everything that Crankshaft can handle in this regard.

R=jarin@chromium.org
BUG=chromium:616709

Review-Url: https://codereview.chromium.org/2067423003
Cr-Commit-Position: refs/heads/master@{#37019}