Don't fail clip-paths with empty bounds

An empty nested clip-path should result in an empty clip-path (clipping
away everything.)

BUG=671543
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2555483003
Cr-Commit-Position: refs/heads/master@{#436605}
4 files changed