Get rid of excess cleverness in benchmark

RotatedRectBench was asking for its base layer size, which may
not be what it expects with odd canvas modes (particularly proxies).
Most benchmarks are not so sophisticated; they hard-wire their
size and just use that (expected) value.

R=mtklein@google.com,djsollen@google.com
BUG=skia:3566

Review URL: https://codereview.chromium.org/1015013004
1 file changed