Field trial for IOSurfaceCapturer on MaxOSX

Add waterfall config for IOSurfaceCapturer experiement.

Bug: 840597
Change-Id: I7530ddb2a2b03ed8eb14a83065f197bfdfab7536
Reviewed-on: https://chromium-review.googlesource.com/1097510Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566579}

testing/variations/fieldtrial_testing_config.json

@@ -1755,6 +1755,21 @@
             ]
         }
     ],
+    "IOSurfaceCapturer": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "IOSurfaceCapturer"
+                    ]
+                }
+            ]
+        }
+    ],
     "ImportantSitesInCBD": [
         {
             "platforms": [