Replace roundedLayoutPoint with LayoutPoint
roundedLayoutPoint is a noop.
And crbug.com/321237 is fixed, so also remove related FIXME.
BUG=
663895
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Committed:
https://crrev.com/c8906d8c65b7ca22d648fdacda5f01af12a11cb9
Cr-Commit-Position: refs/heads/master@{#431408}