Add WebViewHeaderInjection field trial config.

Bug: 1038002
Change-Id: Ib53fe65d5a36af3146ad1f0c61cefbe9afcd8dac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354151
Auto-Submit: Richard Coles <torne@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797718}

testing/variations/fieldtrial_testing_config.json

@@ -8389,6 +8389,21 @@
             ]
         }
     ],
+    "WebViewHeaderInjection": [
+        {
+            "platforms": [
+                "android_webview"
+            ],
+            "experiments": [
+                {
+                    "name": "SameDomainOnly",
+                    "enable_features": [
+                        "WebViewExtraHeadersSameDomainOnly"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebViewOriginCheckForStreamReader": [
         {
             "platforms": [