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In the new UI layout, the PDF area can be resized in two ways

1) expanding/collapsing the sidenav
2) resizing the entire window

Because of #1, the previous approach of listening for the Window's
'resize' event is no longer sufficient. Therefore a ResizeObserver
is used which works for both #1 and #2.

Bug: 1121351
Change-Id: I6fd895a55e4630f9562c8d8ea38dc7d4fc6ebb75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391335Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808469}