[iOS] Merge WKBasedNavigationManagerTests into NavigationManagerTests

Now that there's no longer a WKBasedNavigationManagerImpl, this
CL merges WKBasedNavigationManagerTests into NavigationManagerTests.

Bug: 738020
Change-Id: I0ca38967d64ef8ebade2d808413b478b04c1bf56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581461Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836333}

ios/web/BUILD.gn

@@ -280,7 +280,6 @@ source_set("ios_web_navigation_unittests") {
     "navigation/nscoder_util_unittest.mm",
     "navigation/session_storage_builder_unittest.mm",
     "navigation/wk_back_forward_list_item_holder_unittest.mm",
-    "navigation/wk_based_navigation_manager_impl_unittest.mm",
     "navigation/wk_navigation_action_policy_util_unittest.mm",
     "navigation/wk_navigation_action_util_unittest.mm",
     "navigation/wk_navigation_util_unittest.mm",