Make --install-chrome-app use extensions::WebstoreInstallWithPrompt.

chrome/browser/extensions/startup_helper.h

Index: chrome/browser/extensions/startup_helper.h
diff --git a/chrome/browser/extensions/startup_helper.h b/chrome/browser/extensions/startup_helper.h
index e7829069c0bec1b99337fdfdfbed44a0b79dbee4..971c3ce685cff8d78ccb80455f79708a55d24f2d 100644
--- a/chrome/browser/extensions/startup_helper.h
+++ b/chrome/browser/extensions/startup_helper.h
@@ -53,6 +53,12 @@ class StartupHelper : public PackExtensionJob::Client {
                                   Profile* profile,
                                   base::Callback<void()> done_callback);
 
+  // Handle --install-chrome-app flag from |cmd_line| by downloading metadata
+  // from the webstore for the given id, prompting the user to confirm, and then
+  // downloading the crx and installing it.
+  // Like InstallFromWebstore but this is asynchronous.
+  void InstallChromeApp(const base::CommandLine& cmd_line, Profile* profile);
+
   // Maps the command line argument to the extension id. Returns an empty string
   // in the case when there is no mapping.
   std::string WebStoreIdFromLimitedInstallCmdLine(