[conversions] Add fieldtrial testing config

Enables the ConversionMeasurement browser process feature in tests.
This also enabled the dependent ConversionMeasurement blink runtime
feature in tests as well.

Change-Id: Ie20e39737615f7dbe4ab54fc0275c5848a7b4256
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399514
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806231}

testing/variations/fieldtrial_testing_config.json

@@ -1807,6 +1807,26 @@
             ]
         }
     ],
+    "ConversionMeasurement": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "ConversionMeasurement",
+                    "enable_features": [
+                        "ConversionMeasurement"
+                    ]
+                }
+            ]
+        }
+    ],
     "CrOSMinFilelistKB": [
         {
             "platforms": [