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Slimming paint is now much faster than PicturePile backed recording
for impl-side painting. It's what we're going to ship when we enable
impl-side, so make it the default. Left the disable switch just in case
to try out PicturePile if we need to for some reason until we turn
on impl-side by default.

R=piman@chromium.org, sky
BUG=466426

Review URL: https://codereview.chromium.org/1141583003

Cr-Commit-Position: refs/heads/master@{#329469}