Add flaky expectation for drag-image-descendant-painting-sibling.html

TBR: fs@opera.com
Bug: 1050993
Change-Id: I19a00d98ca20e2b1430cde0dc7640313f4177d33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050166Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740459}

third_party/blink/web_tests/TestExpectations

@@ -447,6 +447,8 @@ crbug.com/1048765 [ Mac10.10 ] external/wpt/svg/animations/animate-marker-orient
 crbug.com/1048765 [ Mac10.10 ] external/wpt/svg/animations/animate-dynamic-update-attributeName.html [ Skip ]
 crbug.com/1048765 [ Mac10.10 ] external/wpt/svg/animations/short-simple-duration-and-fractional-repeatcount.html [ Skip ]
 
+# Slightly flaky timeouts (about 1 in 30)
+crbug.com/1050993 [ Linux ] virtual/gpu-rasterization/images/drag-image-descendant-painting-sibling.html [ Pass Timeout ]
 
 # ====== Paint team owned tests to here ======