Should protect document in ProcessingInstruction::setXSLStyleSheet.
Should protect document in ProcessingInstruction::setXSLStyleSheet.

BUG=403276

Review URL: https://codereview.chromium.org/579133004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@182309 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs