Adding a field trial testing group for LoadingOnlyLearnHighPriorityResources

This experiment shows good results on Android as well as Desktop
platforms, so far. Based on those results, we'll move to Beta for more
data.

Bug: 962693
Change-Id: Id37d846101a2573494b1e340156628ed188dc758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1740650
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685316}

testing/variations/fieldtrial_testing_config.json

@@ -2917,6 +2917,25 @@
             ]
         }
     ],
+    "LoadingOnlyLearnHighPriorityResources": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20190626",
+                    "enable_features": [
+                        "LoadingOnlyLearnHighPriorityResources"
+                    ]
+                }
+            ]
+        }
+    ],
     "LowPriorityIframes2": [
         {
             "platforms": [