Enable Windows NV12 overlays on waterfall

We plan on shipping this via a finch experiment to Canary/Dev soon, and then
enable it by default if we don't encounter major issues. This should only affect
the DirectComposition pixel tests since we force-enable overlays for those
tests. Other tests don't exercise DirectComposition code because those code
paths are disabled unless the hardware supports it.

Bug: 869677
Change-Id: I31c1fd96f2c2567c82c19776a57b2fe18a1b1e1a
Reviewed-on: https://chromium-review.googlesource.com/1170385Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582305}

testing/variations/fieldtrial_testing_config.json

@@ -1373,6 +1373,21 @@
             ]
         }
     ],
+    "DirectCompositionPreferNV12Overlays": [
+        {
+            "platforms": [
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "DirectCompositionPreferNV12Overlays"
+                    ]
+                }
+            ]
+        }
+    ],
     "DownloadHomeMoreButton": [
         {
             "platforms": [