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The original CL used instance variables in a class extension and automatic
generation of ivars for synthesized properties, features only available on
64-bit builds. The Mac Memory bots are still compiling Chromium in 32-bits.
This reland removes the usage of those features.

> This CL adds the class HandoffManager, which is responsible for interfacing
> with Apple's Handoff APIs. It takes a GURL, and exposes that GURL to Handoff.
>
> This CL adds the class ActiveWebContentsObserver, which is responsible for
> listening to changes to the active browser, the active tab, and the visible
> URL. It notifies its delegate when any of this state might have changed.
>
> AppControllerMac is the delegate of ActiveWebContentsObserver, as well as the
> owner of the HandoffManager. When it receives a delegate callback, it passes an
> updated GURL to the HandoffManager. There is some minimal logic in
> AppControllerMac that prevents URLs from incognito windows from being passed to
> the HandoffManager.
>
> BUG=431051, 438823
> Committed: https://crrev.com/708abc5b0abb5e0916d779bf6d1342fd472a2aa1
> Cr-Commit-Position: refs/heads/master@{#307846}

BUG=431051, 438823
TBR=sky, erikwright, mmenke, avi

Review URL: https://codereview.chromium.org/794853004

Cr-Commit-Position: refs/heads/master@{#308005}