Add WebRtcLibvpxEncodeNV12 to field trial testing config

Two variations for applicable platforms (chromeos and mac). On mac, this
feature will only be used in combination with the 0-copy feature
AVFoundationCaptureV2.

Bug: 1134165
Change-Id: Ic630d292041499d0364bf6d7935d278678595f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487103Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/master@{#819364}

testing/variations/fieldtrial_testing_config.json

@@ -7984,6 +7984,22 @@
             ]
         }
     ],
+    "WebRtcLibvpxEncodeNV12": [
+        {
+            "platforms": [
+                "chromeos",
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebRtcLibvpxEncodeNV12"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebSocketReassembleShortMessages": [
         {
             "platforms": [