GitLab

This CL adds a notification via WebContentsDelegate that
notifies the embedder that a new portal WebContents was created.

The chrome/ embedder then uses this notification in order to
attach TabHelpers to the newly created WebContents.

Bug: 865565

Change-Id: I3e39883a5869832e6e59be9db7ea9e88ab18cd64
Reviewed-on: https://chromium-review.googlesource.com/1197143Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592498}