[iOS] Add new ContextMenu in WebViewShell

This CL adds a new boolean to allow the WebViewShell to use the new
ContextMenu from ios/web.
It also removes the handling of the old ContextMenu, which is no longer
used by WebView clients.

Bug: None
Change-Id: Iff30662c90e9351a93a1f93d60ed50df349d3ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3137948


Reviewed-by: John Wu <jzw@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918765}