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Creates a skeletal blink::WebPlugin counterpart to the Pepper-based
OutOfProcessInstance. This will support migrating the PDF viewer from a
Pepper plugin to a plain Blink plugin.

Note that the ultimate intent is _not_ to run the PDF viewer as a plain
Blink plugin, without other changes to isolate it. This is just a
starting point for the final implementation.

Bug: 1085334
Change-Id: I6bea3ddd7fc1b27879af5cd5614071cb02491d77
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328030Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795565}