Add WebRTC-KeepAbsSendTimeExtension to Field Trial testing config.

Feature: https://webrtc-review.googlesource.com/c/src/+/118936

Bug: 925645
Change-Id: Ifac28cbfb3a9f464f14375a53791418a41cfec2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859992
Auto-Submit: Konrad Hofbauer <hofbauer@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706073}

testing/variations/fieldtrial_testing_config.json

@@ -7079,6 +7079,24 @@
             ]
         }
     ],
+    "WebRTC-KeepAbsSendTimeExtension": [
+        {
+            "platforms": [
+                "windows",
+                "mac",
+                "chromeos",
+                "linux",
+                "ios",
+                "android",
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled"
+                }
+            ]
+        }
+    ],
     "WebRTC-LocalIPPermissionCheck": [
         {
             "platforms": [