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kRestoreNavigationTime was a timer created by WKBasedNavigationManagerImpl::Restore
and stopped when session restoration begins navigating to its final URL.

Unfortunately, WKBasedNavigationManagerImpl::Restore is called for every restored
WebState on startup.  Actual network access doesn't happen until a tab is selected. That
means if you have 10 tabs, background/kill/restore, navigate a bit, and then change tabs,
that second tab will have had a timer running since the app started.

Instead, start kRestoreNavigationTime when ios/web begins loading the restore page, and
finish the timer when ios/web begins loading the real page.  This should more correctly
tell how much time SlimNav restoration adds.

Bug: 1005044
Change-Id: I35fee36bb50ec627d7455d6215669a92a2b4bc6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808355
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697757}