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We have an experiment running that would disable parser-inserted usage
of 'chrome-extension://' URLs unless explicitly whitelisted. This breaks
the print preview page, which relies on the PDF extension URLs inside an
HTML import. So. This patch adds a mechanism for whitelisting that
extension URL for the print preview page.

BUG=667224
R=tsepez@chromium.org

Review-Url: https://codereview.chromium.org/2528903002
Cr-Commit-Position: refs/heads/master@{#434770}