Convert FieldTrialActivated to use mojo.

chrome/common/field_trial_recorder.mojom

Index: chrome/common/field_trial_recorder.mojom
diff --git a/third_party/WebKit/Source/modules/csspaint/WindowPaintWorklet.idl b/chrome/common/field_trial_recorder.mojom
similarity index 53%
copy from third_party/WebKit/Source/modules/csspaint/WindowPaintWorklet.idl
copy to chrome/common/field_trial_recorder.mojom
index 0e774ba93ad3e1c58543b915a9f7a154bf16a9d9..007439d7a17735d999af9f9a0bf2ffc98b441263 100644
--- a/third_party/WebKit/Source/modules/csspaint/WindowPaintWorklet.idl
+++ b/chrome/common/field_trial_recorder.mojom
@@ -2,8 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-[
-    RuntimeEnabled=CSSPaintAPI,
-] partial interface Window {
-    readonly attribute Worklet paintWorklet;
+module chrome.mojom;
+
+// Records field trial activations from the renderers.
+interface FieldTrialRecorder {
+  // Indicates that a field trial has been activated.
+  FieldTrialActivated(string trial_name);
 };