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CustomTabActivityTabController

Extracting CustomTabNavigationController will consist of (at least) two
steps:
- Extracting navigate() method
- Extracting back/close navigation

This CL is the first step. Currently the url navigation code is in
CustomTabActivityTabController#loadUrlInTab. In addition to exposing
this public method, the TabController also calls it itself to load the
initial url when the tab is ready. Both of these things are not good
from single responsibility perspective, and need a separate class each.

To achieve that it became necessary to expose the way the initial tab
was created and update the Observer interface correspondingly. To keep
things tidy, holding onto the Tab, the creation method, and observer
management was extracted into CustomTabActivityTabProvider (this is
something we were planning to do eventually anyway).

In this CL the code becomes structured as follows:

TabController - minds the tab creation and initialization business, no
longer navigates to any urls. Updates TabProvider (via package-local
methods).

TabProvider - hold the current tab and information about how the
initial tab was created. Dispatches changes to observers. Typically,
the features should only need to use TabProvider, but not
TabController.

NavigationController - navigates to urls. Will also contain back/close
navigation in future.

InitialPageLoader - observes TabController and commands
NavigationController to navigate to the initial url.

Bug: 916528
Change-Id: Ic26ef955ab8e3e0bd926461c71ef740361686f17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530884
Commit-Queue: Pavel Shmakov <pshmakov@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645309}