[PE] Fix subpixel accumulation for composited content layers

The content box rect was improperly snapped in the local space.
We should snap in the layout space instead, including subpixel
accumulation.

This is a duplicate of trchen@'s old CL
https://codereview.chromium.org/2172503002/ rebased on the current
trunk.

Bug: 627683
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I737871c4bafead28f1040d486a59f3203c0b0ead
Reviewed-on: https://chromium-review.googlesource.com/c/1271582
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600811}