Temporarily supppress additional canvas 2D rectangle shadow test

Bug: 990918
Change-Id: Iba9863e480e93a4618e56f8f1998327a0e03c5f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758912Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#688569}

third_party/blink/web_tests/TestExpectations

@@ -5250,6 +5250,7 @@ crbug.com/775957 virtual/gpu/fast/canvas/canvas-incremental-repaint.html [ Skip
 # Canvas tests that are affected by change to Skia GPU rect shadow rendering.
 crbug.com/990918 virtual/gpu/fast/canvas/canvas-shadow-source-in.html [ Pass Failure ]
 crbug.com/990918 virtual/gpu/fast/canvas/canvas-composite-shadow.html [ Pass Failure ]
+crbug.com/990918 virtual/gpu/fast/canvas/canvas-shadow.html [ Pass Failure ]
 
 # Sheriff failures 2017-09-21
 crbug.com/767469 http/tests/navigation/start-load-during-provisional-loader-detach.html [ Pass Failure ]