Streamline GraphicsContext::drawRect()

Currently, drawRect() is emulating an inset border by filling 4
different 1-sized rects. We can attain the same result by stroking one
inset rect.

One pixel test requires minor rebaselining.

R=senorblanco@chromium.org, schenney@chromium.org

Review URL: https://codereview.chromium.org/185653008

git-svn-id: svn://svn.chromium.org/blink/trunk@168671 bbb929c8-8fbe-4397-9dbb-9b2b20218538