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Controlled by --enable-features=PdfIsolation, default is off.
Stricter than required (full origin, not eTLD+1).
Add pdf_extension_test for multiple domains.
A subsequent patch may attempt to apply this to secure origins only.

Bug: 809614

Change-Id: Ic38c2c5074c7723210e38a754b930fb152c031b7
Reviewed-on: https://chromium-review.googlesource.com/953979Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542162}