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Adds a "LayoutOptions" object that is passed from the PDF plugin (via
the "documentDimensions" message), through the extension, and back to
the plugin (via the "viewport" message).

We plan to make layout asynchronous for crbug.com/885110. To accomplish
this, we need to carry certain layout state through the JavaScript
layer, until the layout change finally is applied.

In the future, we may want to modify the layout options in JavaScript
(such as for 2-up view), but for now, we just need to pass the exact
same object back and forth.

Bug: 885110
Change-Id: Iaac57422dbba185399358ceffdbde4e6ad2c7899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815769
Commit-Queue: K Moon <kmoon@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700417}