Add --bbh (default true) to nanobench.

Chrome's using a bounding box, so it's a good idea for our
bots to do so too.

When set, we'll create an SkTileGrid to match the
parameters of --clip, and so should always hit its fast
path.

This will impose a small overhead (querying the BBH) on all
SKPs, but make large SKPs render more quickly.  E.g. on
GPU desk_pokemonwiki should show about a 30% improvement,
tabl_mozilla about 40%, and one very long page from my
personal suite, askmefast.com, gets 5x faster.

(The performance changes are not the point of the CL, but
something we should be aware of.)

BUG=
R=bsalomon@google.com, mtklein@google.com, robertphillips@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/497493003

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