Enable field trial test for JPEG YUV decoding.

Add field trial entry for decoding JPEG 4:2:0 images to YUV.

Bug: 919627
Change-Id: Ie8dc6378cccf27527da9ea05d060ba218fb69bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898548Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Sasha McIntosh <sashamcintosh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715882}

testing/variations/fieldtrial_testing_config.json

@@ -1819,6 +1819,26 @@
             ]
         }
     ],
+    "DecodeJpegToYuv": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Experiment",
+                    "enable_features": [
+                        "DecodeJpeg420ImagesToYUV"
+                    ]
+                }
+            ]
+        }
+    ],
     "DecodeWebPToYUV": [
         {
             "platforms": [