Revert of Remove unneeded border fastpath check
Revert of Remove unneeded border fastpath check

(manual revert of http://crrev.com/1093523002)

Reason for revert: noticeable rasterize time regression for some mobile
pages/devices.

Drawing via fillPath() turns out to be slower than the normal complex
border path - except when it avoids creating transparency layers. Hence
the border alpha test.

Reverting the change and adding a comment.

BUG=478796
TBR=junov@chromium.org

Review URL: https://codereview.chromium.org/1109393009

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194691 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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