Add web_manifest_url in WebappInfo.

chrome/android/webapk/shell_apk/BUILD.gn

Index: chrome/android/webapk/shell_apk/BUILD.gn
diff --git a/chrome/android/webapk/shell_apk/BUILD.gn b/chrome/android/webapk/shell_apk/BUILD.gn
index 87fb83537f57ad34d364508dddb7466b6819b5e0..27c708d49b096e6c6e3b427882f251b2d0dff7d9 100644
--- a/chrome/android/webapk/shell_apk/BUILD.gn
+++ b/chrome/android/webapk/shell_apk/BUILD.gn
@@ -12,6 +12,9 @@ declare_args() {
   # The browser that the WebAPK will be bound to.
   webapk_runtime_host = "com.google.android.apps.chrome"
 
+  # The Url of the Web Manifest file.
+  webapk_web_manifest_url = "https://www.template.com/manifest.json"
+
   # Attributes from Web Manifest.
   webapk_start_url = "https://www.template.com/home_page"
   webapk_name = "Longer Sample WebAPK Name"
@@ -51,6 +54,7 @@ jinja_template("shell_apk_manifest") {
     "background_color=$webapk_background_color",
     "icon_url=$webapk_icon_url",
     "scope_url_host=$webapk_scope_url_host",
+    "web_manifest_url=$webapk_web_manifest_url",
   ]
 }