Use clipRRect instead of clipPath to clip out rounded rects.

Changes clipping out of rounded rects to use clipRRect instead generic
clipPath. This is particularly important for GPU rasterization, where ops
with generic paths are expensive.

Note that this will change the results of 67 layout tests. This is to be
expected, because Skia's RRect uses analytic circles for the rounded corners,
instead of an approximation by splines.

R=esprehn@chromium.org
BUG=343735

Review URL: https://codereview.chromium.org/165543003

git-svn-id: svn://svn.chromium.org/blink/trunk@167308 bbb929c8-8fbe-4397-9dbb-9b2b20218538