blink:bindings: Passes the global context instead of |this| in JS.

|this| in JS (args.Holder() in C++ code) is not a reliable object.  User
script can pass any object as |this|.  So we shouldn't use it as creation
context when calling Blink APIs to create a new DOM wrapper.

We should instead use the current context where the user script is running
as creation context.

BUG=497507

Review URL: https://codereview.chromium.org/1174343003

Cr-Commit-Position: refs/heads/master@{#334366}