Reland "Portals: Support activating a portal."

This is a reland of 831290e17ad0fccc4af70516218bf35833d88ac7

Original change's description:
> Portals: Support activating a portal.
>
> When a portal is activated, the portal contents replace the tab contents.
>
> Bug: 865565
> Change-Id: Id666e94b0c98294dbc2eb1ce942101a2f158ebf3
> Reviewed-on: https://chromium-review.googlesource.com/c/1188884
> Commit-Queue: Lucas Gadani <lfg@chromium.org>
> Reviewed-by: Nasko Oskov <nasko@chromium.org>
> Reviewed-by: Jeremy Roman <jbroman@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#596428}

Tbr: avi@chromium.org,jbroman@chromium.org,yfriedman@chromium.org
No-Presubmit: true

Bug: 865565
Change-Id: I0df13476e8e95a035897f38884123019150cd7b8
Reviewed-on: https://chromium-review.googlesource.com/c/1262090
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596834}
28 files changed