Mark jpeg-yuv-image-decoding.html as flaky

NOTRY=true

Bug: 954994
Change-Id: I5a91e3fd45cb886990ab103911117285a7f6a813
TBR: mbarowsky@chromium.org, wangxianzhu@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577496Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652874}

third_party/blink/web_tests/TestExpectations

@@ -6158,3 +6158,8 @@ crbug.com/953994 [ Linux Win ] virtual/outofblink-cors/http/tests/security/svg-i
 
 # Sheriff 2019-04-19
 crbug.com/954628 [ Win ] external/wpt/payment-request/payment-request-canmakepayment-method.https.html [ Pass Timeout ]
+
+# Sheriff 2019-04-22
+crbug.com/954994 [ Mac ]  images/jpeg-yuv-image-decoding.html [ Pass Failure ]
+crbug.com/954994 [ Mac ]  virtual/exotic-color-space/images/jpeg-yuv-image-decoding.html [ Pass Failure ]
+crbug.com/954994 [ Mac ]  virtual/gpu-rasterization/images/jpeg-yuv-image-decoding.html [ Pass Failure ]