Introduce a new WebAPK launch action.

chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java

Index: chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
diff --git a/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java b/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
index 693cb384e5b99553502f4d6c03a657d502e3e931..d33e4d30516ea2467679e10225bd40bb9e8a288a 100644
--- a/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
+++ b/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
@@ -38,8 +38,9 @@ public class MainActivity extends Activity {
     private static final String HOST_BROWSER_LAUNCHER_CLASS_NAME =
             "org.chromium.webapk.lib.runtime_library.HostBrowserLauncher";
 
-    // Action for launching {@link WebappLauncherActivity}. Must stay in sync with
-    // {@link WebappLauncherActivity#ACTION_START_WEBAPP}.
+    // Action for launching {@link WebappLauncherActivity}.
+    // TODO(hanxi): crbug.com/737556. Replaces this string with the new WebAPK launch action after
+    // it is propagated to all the Chrome's channels.
     public static final String ACTION_START_WEBAPK =
             "com.google.android.apps.chrome.webapps.WebappManager.ACTION_START_WEBAPP";