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Before this CL, GetPendingItemIndex() used to return last committed item
index for the new navigations (none-reload, none-back-forward).
Presumably this was done before CRWSessionController implemented
pendingItemIndex support, when CRWSessionController immediately changed committed item
during back-forward navigations. pendingItemIndex support design doc: https://docs.google.com/document/d/1GvsaoNtMO7d87FzoysOW5GqH6XErI5g2cinKhkPk7bQ/edit

This CL returns -1 for new navigations, which matches //content.

There are 3 places where GetPendingItemIndex is used outside of ios/web:
 - IOSLiveTab::GetPendingEntryIndex (new implementation matches content,
   which is the right approach)
 - GetPossiblyPendingItemAtIndex (new implementation should have no
   effect because index should be in [0;size] bounds, but this is just
   an assumption and it's actually unclear if GetPossiblyPendingItemAtIndex
   relied on undocumented and incorrect behavior).
 - ReadingListWebStateObserver::LoadOfflineReadingListEntry (new
   implementation should make the whole block of code obsolete, but
   it should be fine to leave it as it is because
   ReadingListWebStateObserver is going to be removed soon).

Bug: 665189, 899827
Change-Id: I9538b693c924790620140b37d6c4fea6cce26fde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504231
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638637}