Reland "Portals: Expose portalHost to portal's browsing context"

This is a reland of 9d4738420905a6b28407a8282512d8307d413780

Original change's description:
> Portals: Expose portalHost to portal's browsing context
> 
> window.portalHost returns a PortalHost to script running inside the main
> frame within a portal.
> 
> Bug: 914117
> Change-Id: I26fb4ae9676261bee4446bfbc6ee34b28dc618b1
> Reviewed-on: https://chromium-review.googlesource.com/c/1399569
> Commit-Queue: Adithya Srinivasan <[email protected]>
> Reviewed-by: Daniel Cheng <[email protected]>
> Reviewed-by: Charlie Reis <[email protected]>
> Reviewed-by: Lucas Gadani <[email protected]>
> Reviewed-by: Jeremy Roman <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#624316}

Bug: 914117
Change-Id: I2830cafac26928229d474fd91921fbf2f9a47952
Reviewed-on: https://chromium-review.googlesource.com/c/1425041
Commit-Queue: Adithya Srinivasan <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Reviewed-by: Jeremy Roman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#628329}
21 files changed