Create and use BlinkInterfaceProvider WeakPtrs on the same thread

Create and use BlinkInterfaceProvider WeakPtrs on the same thread

A race condition seems to be occurring where the reference to
WeakReference::Flag in WeakReferenceOwner is in process of being destroyed
while a new reference is being added. This happens when a document having
Web Workers is being destroyed (due to a reload) while, on the thread
specific to a worker, an API is used that binds to a Mojo Interface
Provider.

Creating the weak pointers on arbitrary threads, even though they
consistently get dereferenced on the main thread, seems to be unsafe
in this situation.

Instead, create a single WeakPtr<> on the main thread during construction,
a copy of which can be made on the worker threads. This will increase the
reference count of the WeakReference::Flag, which is a thread-safe ref
counted object.

BUG=649645
Review-Url: https://codereview.chromium.org/2476543003
Cr-Commit-Position: refs/heads/master@{#430663}
(cherry picked from commit c6f247ed04bc2e28157968a9f7e8f195012ae614)

[Peter Beverloo, 2016-11-10 23:00:36]

Committed patchset #1 (id:1) to pending queue manually as
ef2701435a566dc72d270a8d3ae7d8e2e558ecd8.