Remove unused declaration of processViewport in Document
Remove unused declaration of processViewport in Document

Trivial clean-up, the function was removed in crrev.com/23606013 but the
declaration was left in by accident.

R=dstockwell
BUG=

Review URL: https://codereview.chromium.org/108683005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@163998 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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