Remove unused paint offset from PaintBoxDecorationBackgroundWithRect

Working on PaintTouchActionRects, I found this is unused. Removing this
makes the code cleaner.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I762933aac6c937b8f7f617eb047fffbf4ddd3123
Reviewed-on: https://chromium-review.googlesource.com/1150829
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578374}
3 files changed