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The referenced bug occurs due to ordering issues of our viewport inset
updates relative to PageDisplayState application.  When reloading the
page using overscroll, the PageDisplayState is stored.  Since the page
is scrolled to the top, the contentOffset is -contentInset.top (-92 in
portrait on iPhone X).  Upon rotating the device, the following events
occur:
1) CRWWebController's |-orientationDidChange| kicks off the scroll
   restoration process for the y offset of -92.  This is asynchronous
   because it first executes JavaScript in order to extract the
   viewport tag.
2) BVC's |-traitCollectionDidChange:| updates the toolbar heights for
   the new horizontal size class, which in turn updates the scroll
   view's contentInset.  For landscape iPhone X, this produces a top
   contentInset of 50 (i.e. the "scrolled to top" y content offset is
   -50).
3) The asynchronous viewport meta tag data extraction finishes and
   CRWWebController restores the -92 contentOffset, pushing the page
   below the toolbar.

This CL updates PageScrollState semantics such that it stores the
offset from the scrolled-to-top-left position, which allows the
application of PageScrollStates contentInset-agnostic.

Bug: 923856
Change-Id: I468970abbdbd200e4ac836d5664a985f0c8e3e71
Reviewed-on: https://chromium-review.googlesource.com/c/1432377Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625828}