Add fieldtrial_testing_config for VaapiH264AMDEncoder.

This CL adds a fieldtrial testing config for VaapiH264AMDEncoder,
that plans to turn on for all versions 81+.

Bug: 1035788
Test: None
Change-Id: Ia7d7b41da72e998006c30a3f8e1f77bd6e11d11a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040492
Auto-Submit: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738860}

testing/variations/fieldtrial_testing_config.json

@@ -7234,6 +7234,21 @@
             ]
         }
     ],
+    "VaapiH264AMDEncoder": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "VaapiH264AMDEncoder"
+                    ]
+                }
+            ]
+        }
+    ],
     "VideoCaptureService": [
         {
             "platforms": [