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

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}
(cherry picked from commit c093b7a74ddce32dd3b0e0be60f31becc6ce32f9)

[haraken, 2016-12-14 01:46:56]

Committed patchset #1 (id:1) to pending queue manually as
3e810541d47ed6852e9fefde94c494fb8677c1a8.