Perform SkPicture analysis lazily.

I realized when writing the comment on https://crrev.com/1135363002/
that I'd really just sketched out the entire thing, so I couldn't help
but actually write up a working CL.  How does this do for your benchmark?

BUG=chromium:487075

Review URL: https://codereview.chromium.org/1130123006
2 files changed