[iOS Clean] Wired up ContextMenuCommands.

This CL updates WebContextMenuCoordinator and its associated mediator
class to support more actions.  It will load URLs and create new
WebStates, but still does not perform corresponding animations to show
newly created Tabs and their loaded content.

BUG=none

Review-Url: https://codereview.chromium.org/2862783002
Cr-Commit-Position: refs/heads/master@{#475257}