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All mojo interfaces that manipulate NSViews need to be associated (for
example, adding an NSView as a child of another requires that the
parent NSView be in a known state -- each NSView has its own interface,
and so they all need to be associated to the same pipe).

Also change all bindings to use ui::WindowResizeHelperMac's task
runner. This task runner is used to pump a nested run loop inside
a Cocoa resize function (for smooth resize). All bindings on the same
associated connection need to use that task runner (otherwise, all
messages will be blocked behind any un-handled message).

Change BridgeFactoryHost to not assign a new host id, but take one
in the constructor (just because the logic is easier to follow that
way).

R=domonickn
TBR=avi (for content/ OWNERship)

Bug: 821651
Change-Id: Ifb6aff292187b340d144d29ffe1258bb8d501267
Reviewed-on: https://chromium-review.googlesource.com/c/1253561
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596013}