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This uncovered a bug where GetLastCommittedItemIndex() returned the
incorrect value because it assumes WKBackForwardList's |currentItem|
is always the last committed item. This is not the case if the pending
navigation is a back-forward navigation. It turned out that this can't
be fixed 100%, but implemented a heurstic and documented the edge case.

Bug: 734150
Change-Id: Ie2c026a2c5231cbd4151c106cd0db664dcf6d587
Reviewed-on: https://chromium-review.googlesource.com/596828Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491749}