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web::features::UseWKWebViewLoading is a helper method that uses 3
related feature flags to control the new feature of using
WKWebView.loading for WebState::IsLoading:

1. kSlimNavigationManager is used because we only want to fix the
   failing test cases with slim-nav enabled;
2. kLogLoadStartedInDidStartNavigation is used because the CPM metric
   was logged in WebStateDidStartLoading. This flag moves the logging to
   WebStateDidStartNavigation, and if it's enabled then the CPM won't be
   affected by this feature;
3. kUseWKWebViewLoading is the flag for this feature, because we want
   to use it for experiment with kLogLoadStartedInDidStartNavigation
   enabled to make sure that this feature doesn't change CPM.

Bug: 991763
Change-Id: I99fe31317277163eb4768b2db1c9ef8f5d2dd844
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833470Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708135}