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Add ToVar() and FromVar() methods to DocumentLayout::Options, allowing
serialization to and from a pp::Var. This serialization is then used to
pass layout options (such as default page orientation) to and from
JavaScript (via the "documentDimensions" message).

pp::Var is non-trivial to unit test, due to dependencies on the
browser's PPAPI interfaces. Instead, this change adds 2 browser_tests,
PDFExtensionTest.Layout3 and PDFExtensionTest.Layout4. Each runs the
same layout_test.js suite over two slightly different test PDF files,
test-layout3.pdf (with 3 pages) and test-layout4.pdf (with 4 pages).

There's currently no coverage of deserialization, as we're not doing
anything with the layout options sent back from JavaScript yet. We'll
start using FromVar() in an upcoming CL (such as crrev.com/c/1747170).

Bug: 885110
Change-Id: I8c93b74062576ee07445070e0b4a82f0f861a82e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830398Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: K Moon <kmoon@chromium.org>
Auto-Submit: K Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702324}