Don't pixel-snap box size before applying percentage transform to it.
Otherwise we'll end up with error that gets larger according to the percentage.
Note that subpixel accumulation is already plumbed through for transforms in
other ways.
BUG=
627588
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Committed:
https://crrev.com/234079c8f5478894d70b6899a7461caa7062504d
Cr-Commit-Position: refs/heads/master@{#422680}