HTMLDocumentParser: Don't access document() before detach check
HTMLDocumentParser: Don't access document() before detach check

document() may be null if parser isDetached().

Move the access after isStopped() bail-out so that document() is guaranteed
to be valid.
Note: !isStopped() ensures that !isDetached()

BUG=478662

Review URL: https://codereview.chromium.org/1091953003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194476 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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