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Currently PDFiumEngine::GetPageScreenRect() calculates the area
of the |page_index| page, including the area around it such as
shadows and separators. In two-up view, when two pages are
side-by-side and one page is shorter than the other, the space
below the shorter page must be drawn as empty space to match
the height of the taller page.

This CL refactors PDFiumEngine::GetPageScreenRect() so that when
|two_up_view_| is true, it calls draw_utils::GetSurroundingRect()
with a copy of the rect for |page_index| page with its height set
to the max height of the two pages. It also adds
PDFiumEngine::GetAdjacentPageIndexForTwoUpView() which retrieves
the index of the page adjacent to the current page.

Bug: 51472
Change-Id: I869903f869e926117cd60beb92db0ce96606579a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715116
Commit-Queue: Jeremy Chinsen <chinsenj@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680271}