Import web-platform-tests/css/css-masking/

These tests are already doing fairly well on Chrome:
https://wpt.fyi/css/css-masking

However, there's also at least one Chrome-only failure:
https://wpt.fyi/css/css-masking/parsing/clip-path-invalid.html

Note that some of the tests are still problematic:
https://wpt.fyi/css/css-masking/clip-path
https://github.com/w3c/web-platform-tests/issues/7543#issuecomment-388744004

No-Try: true
Change-Id: I036e08fd44b12291f8fcce386b67b7d5897365da
Reviewed-on: https://chromium-review.googlesource.com/1057090Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558643}

third_party/WebKit/LayoutTests/W3CImportExpectations

@@ -86,7 +86,6 @@ external/wpt/css/css-counter-styles [ Skip ]
 external/wpt/css/css-display/run-in [ Skip ]
 external/wpt/css/css-exclusions [ Skip ]
 external/wpt/css/css-gcpm [ Skip ]
-external/wpt/css/css-masking [ Skip ]
 external/wpt/css/css-page [ Skip ]
 external/wpt/css/css-regions [ Skip ]
 external/wpt/css/css-round-display [ Skip ]

third_party/WebKit/LayoutTests/external/wpt/css/css-masking/OWNERS

+# TEAM: paint-dev@chromium.org
+# COMPONENT: Blink>Paint
+# WPT-NOTIFY: true