Create field trial for hover card images.

This is required to deploy a finch configuration.
See cr/296239960 for WIP Finch config.

Bug: 1032640
Change-Id: Ifae3d2966d4ca908982cec37c5d88f786bf07367
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067558Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743826}

testing/variations/fieldtrial_testing_config.json

@@ -5948,6 +5948,24 @@
             ]
         }
     ],
+    "TabHoverCardImages": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "TabHoverCardImages"
+                    ]
+                }
+            ]
+        }
+    ],
     "TabHoverCards": [
         {
             "platforms": [