Use rounded rects for rectangular focus rings.

Focus rings are currently painted using paths + SkCornerPathEffects. The
latter prevents Ganesh rasterization (not supported).

But most focus rings are rectangular -> we can avoid building an SkPath
+ SkCornerPathEffect, and draw them directly as RRects instead.

BUG=398635
R=schenney@chromium.org,jbroman@chromium.org,reed@google.com,senorblanco@chromium.org

Review URL: https://codereview.chromium.org/430023002

git-svn-id: svn://svn.chromium.org/blink/trunk@179475 bbb929c8-8fbe-4397-9dbb-9b2b20218538