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I added the UIPage layer which is the shared structure between all the
WebUI pages. This involved fleshing out the ButtonNavigationBehavior to
allow links appearing in the UIPage to have page-specific customized
callbacks for links appearing in the messages (and the behavior was
accordingly renamed UiPageContainerBehavior).

To make sure that the structure was functional, I added links into the
placeholder text for the pages' messages where they are supposed to
appear in the actual text and tested that they can access custom
callbacks.

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I4963c9e573e83ea4c699920639cfe197fe6e1613
Reviewed-on: https://chromium-review.googlesource.com/1029531Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jordy Greenblatt <jordynass@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554203}