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Currently, we're sending `Sec-Fetch-Dest: unknown` for PPAPI-driven
requests. We ought to be sending `embed` or `object`.

This patch adds some tests to ensure that we're setting the context
for PPAPI requests to PLUGIN, and changes Blink's behavior to report
PLUGIN as `embed`. We ought to be distinguishing between `embed` and
`object`, but don't currently have the context when creating the
request to make that possible (and "embed" is strictly better than
"unknown" :) ).

Bug: 1021432
Change-Id: Ie0d2dff1dee85e34c2fab6f7ea2e542fede644e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899446Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712911}