Don't touch the prototype chain to get the private script controller.

Prior to this patch, private scripts attempted to get the
"privateScriptController" property off the global object without verifying if
the property actually exists on the global. If the property hasn't been set yet,
this operation could descend into the prototype chain and potentially return
a named property from the WindowProperties object, leading to release asserts
and general confusion.

BUG=668552

Review-Url: https://codereview.chromium.org/2529163002
Cr-Commit-Position: refs/heads/master@{#434627}