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1. Add a two-up view flag inside DocumentLayout::Options. Modify
   layout_test.js accordingly and add matching unit tests.
2. Add backend support for handling message type "setTwoUpView",
   which will update viewport's layout options and apply the changes.
3. Add unit tests for PDFiumEngine::SetTwoUpView().

Bug: 51472
Change-Id: I212418b439e860feb9e6ed8761a5f722f2a14a9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976156Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Hui Yingst <nigi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732517}