Prevent Previews from interfering with Offline Pages

Some previews can cause problems for offlining. We should disable
those previews.  This approach uses a NavigationDelegate and a
WebContentsDelegate to allow the background offliner to disable
previews that could cause problems for offlining.

Bug: 703875
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I519fae1b6f98da99c89ff97cb4b201c60a92b176
Reviewed-on: https://chromium-review.googlesource.com/544586
Commit-Queue: Peter Williamson <[email protected]>
Reviewed-by: Charlie Harrison <[email protected]>
Reviewed-by: Cathy Li <[email protected]>
Reviewed-by: Ryan Sturm <[email protected]>
Reviewed-by: Alex Moshchuk <[email protected]>
Cr-Commit-Position: refs/heads/master@{#485014}
17 files changed