Enforce JPEG YUV decoding in gpu rastererization web tests.

This flag shoulld already be enabled for virtual web tests, since
crrev.com/c/1798523

Additionally disable JPEG YUV decoding for non-yuv gpu rasterization
tests.

Bug: 1037798
Change-Id: I80c8ea7eb54478b54f14145f18fb1b90023db395
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1982628Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Sasha McIntosh <sashamcintosh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730221}

third_party/blink/web_tests/VirtualTestSuites

@@ -68,13 +68,15 @@
     "prefix": "gpu-rasterization",
     "bases": ["images"],
     "args": ["--force-gpu-rasterization",
-             "--enable-features=DecodeLossyWebPImagesToYUV"]
+             "--enable-features=DecodeLossyWebPImagesToYUV",
+             "--enable-features=DecodeJpeg420ImagesToYUV"]
   },
   {
     "prefix": "gpu-rasterization-disable-yuv",
     "bases": ["images/yuv-decode-eligible"],
     "args": ["--force-gpu-rasterization",
-             "--disable-features=DecodeLossyWebPImagesToYUV"]
+             "--disable-features=DecodeLossyWebPImagesToYUV",
+             "--disable-features=DecodeJpeg420ImagesToYUV"]
   },
   {
     "prefix": "exotic-color-space",