Add WebView UMA log package name to field trial testing config.

Bug: 839646
Change-Id: I0b28b079bd5e44d8f8d9a6057d2740b4f6bc9bcb
Reviewed-on: https://chromium-review.googlesource.com/c/1370373Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615807}

testing/variations/fieldtrial_testing_config.json

@@ -5629,6 +5629,21 @@
             ]
         }
     ],
+    "WebViewUmaLogAppPackageName": [
+        {
+            "platforms": [
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "WebViewUmaLogAppPackageName",
+                    "enable_features": [
+                        "WebViewUmaLogAppPackageName"
+                    ]
+                }
+            ]
+        }
+    ],
     "WinOOPSelectFileDialog": [
         {
             "platforms": [