Disable failing PDFExtensionTests on Site Isolation FYI bots.
Disable failing PDFExtensionTests on Site Isolation FYI bots.

TBR=jam
BUG=498335,417518
TEST=Site Isolation FYI bots go green.

Review URL: https://codereview.chromium.org/1169153002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#333531}

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