[Sheriff] Update expectations for Pixel_PrecisionRoundedCorner test.

It is flaking on Linux, Windows and Mac.
Marked as RetryOnFailure.

Bug: 1028975
Change-Id: I631f50dce9d4836fffbe1852aeb5a130dbdc666c
No-Try: True
TBR: malaykeshav@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943175Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720162}

content/test/gpu/gpu_tests/test_expectations/pixel_expectations.txt

@@ -241,4 +241,9 @@ crbug.com/1019462 [ android nvidia no-skia-renderer ] Pixel_WebGL2_BlitFramebuff
 crbug.com/681341 [ mac ] Pixel_WebGLLowToHighPower [ Skip ]
 
 # Flakes on Mac Retine NVIDIA producing a blank image.
-crbug.com/1027518 [ mac nvidia ] Pixel_Video_Context_Loss_VP9 [ Skip ]
\ No newline at end of file
+crbug.com/1027518 [ mac nvidia ] Pixel_Video_Context_Loss_VP9 [ Skip ]
+
+# Flakes on Linux, Mac and Windows
+crbug.com/1028975 [ linux ] Pixel_PrecisionRoundedCorner [ RetryOnFailure ]
+crbug.com/1028975 [ mac ] Pixel_PrecisionRoundedCorner [ RetryOnFailure ]
+crbug.com/1028975 [ win ] Pixel_PrecisionRoundedCorner [ RetryOnFailure ]