[Fuchsia] Use fuchsia.ui.gfx.ExportToken to pass view export token

1. Updated web.Frame.CreateView() to take ExportToken - all callers
   should be using CreateView2, so it's safe to update this method.
2. Updated all other places that pass view tokens to use ExportToken
   type instead of zx::eventpair.
3. Removed viewsv1::ViewProvider implementation from WebComponent.
4. Removed other vestigial dependencies on viewsv1.

Some APIs still use eventpair instead of ExportToken - they will be
updated later.

Bug: 899348
Change-Id: Ic08485573e6d75e1b45249467a9eb5a57f046057
Reviewed-on: https://chromium-review.googlesource.com/c/1471227
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632311}
15 files changed