Revert "Portals: Support activating a portal."

This reverts commit 831290e17ad0fccc4af70516218bf35833d88ac7.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 596428 as the
culprit for flakes in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/flake/flake-culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vODMxMjkwZTE3YWQwZmNjYzRhZjcwNTE2MjE4YmYzNTgzM2Q4OGFjNww

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-rel/14218

Sample Failed Step: viz_content_browsertests

Sample Flaky Test: PortalBrowserTest.ActivatePortalInShell

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}

Change-Id: I8700b1340612578081c2b3ccd12a57ff48f0fd84
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 865565, 892004
Reviewed-on: https://chromium-review.googlesource.com/c/1261595
Cr-Commit-Position: refs/heads/master@{#596601}
28 files changed