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CRWWebController sometimes needs to remove WKWebView (e.g. crbug/770914)
and this obliterates the session history when WKBasedNavigationManager
is used. This CL adds the ability for WKBasedNavigationManager to
temporarily be detached from the web view.

Major parts to this CL:
1. NavigationManagerImpl::DetachFromWebView() is a new API that
   CRWWebController uses to notify the navigation manager before
   removing the web view.
2. WKBasedNavigationManager implements this API with the help of a new
   nested class WKWebViewShim. It knows how to cache the items when
   detaching from a web view and service getter calls in detached mode.
3. Added overrides for FinishReload(), FinishLoadURLWithParams() and
   LoadIfNecessary() in WKBasedNavigationManager so it can switch back
   to attached mode when a new navigation starts by restoring the cached
   navigation items.

Also cleaned up unneeded APIs from CRWWebViewNavigationProxy.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2b1796731964f021df8c0489d3a6ef56ae390e04
Bug: 815248
Reviewed-on: https://chromium-review.googlesource.com/999054
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549880}