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

Review-Url: https://codereview.chromium.org/2386263002
Cr-Commit-Position: refs/heads/master@{#422680}
9 files changed