[Android WebView] Update user agent to match current trunk builds.

Review URL: https://codereview.chromium.org/670713002

Cr-Commit-Position: refs/heads/master@{#300480}

android_webview/common/aw_content_client.cc

@@ -18,7 +18,7 @@ std::string GetProduct() {
   // "Chrome/XX.0.0.0" identifies that this WebView is derived from the
   // corresponding Chromium version XX.
   // TODO(torne): Use chrome/VERSION file. See http://crbug.com/297522
-  return "Chrome/38.0.0.0";
+  return "Chrome/40.0.0.0";
 }
 
 }