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For a XYZ fit type named destination, if the x or y coordinate is null,
that specific coordinate of the current viewport should be retained (See
table 151 in ISO 32000-1 for more details.). Also, one null parameter
does not invalidate other parameters, all parameters should be handled
independently.

Currently, the PDF viewer ignores both the x and y coordinates if either
one of them is null, and it always resets the viewport position to (0,0)
(top-left corner) without retaining the current coordinate which receives
a null input.

This CL does the following changes to fix this issue:
- Makes PDFiumPage::GetPageDestinationTarget() return x and y
  coordinates separately, so that each null input can be handled
  individually as an undefined value.
- Introduces retrieveCurrentScreenCoordinates() to calculate the current
  in-screen coordinates of the viewport, so that goToPageAndXY() can use
  its return value to retain x/y coordinate while navigating to a new
  destination.

Bug: 1163701,1157061
Change-Id: I4a4e8f1e193aab8ad2d5c8f3a277d0892e3e72c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632325Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844663}