Implement declarative content script API for <webview> [js part].

chrome/test/data/extensions/platform_apps/web_view/shim/main.js

Index: chrome/test/data/extensions/platform_apps/web_view/shim/main.js
diff --git a/chrome/test/data/extensions/platform_apps/web_view/shim/main.js b/chrome/test/data/extensions/platform_apps/web_view/shim/main.js
index 4778664caca6ef91b607e19702baebf3d8fd0f18..01cc78490f40f1b34095756c050fd34e75f1b8b1 100644
--- a/chrome/test/data/extensions/platform_apps/web_view/shim/main.js
+++ b/chrome/test/data/extensions/platform_apps/web_view/shim/main.js
@@ -513,7 +513,9 @@ function testWebRequestAPIExistence() {
     'onBeforeRedirect',
     'onResponseStarted',
     'onCompleted',
-    'onErrorOccurred'
+    'onErrorOccurred',
+    // Declarative Content API.
+    'onPageChanged'
   ];
   var webview = document.createElement('webview');
   webview.setAttribute('partition', arguments.callee.name);