arc: Handle default app not availble case.

components/arc/common/app.mojom

Index: components/arc/common/app.mojom
diff --git a/components/arc/common/app.mojom b/components/arc/common/app.mojom
index 6aedc0dacf180e35f341ff4ccd4b2f7f9fbb5004..839dda6c10cd47bf34bf756f2c0a0fca6cd3924d 100644
--- a/components/arc/common/app.mojom
+++ b/components/arc/common/app.mojom
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
-// Next MinVersion: 17
+// Next MinVersion: 18
 
 module arc.mojom;
 
@@ -31,6 +31,12 @@ enum OrientationLock {
   LANDSCAPE_SECONDARY = 7,
 };
 
+// Describes installation result.
+struct InstallationResult {
+  string package_name;
+  bool success;  // true if app was installed successfully.
+};
+
 // Describes ARC app.
 struct AppInfo {
   string name;
@@ -129,11 +135,15 @@ interface AppHost {
   // Notifies that an application shortcut needs to be created.
   [MinVersion=9] OnInstallShortcut@11(ShortcutInfo shortcut);
 
-  // Notifies that Play Store installation has been started.
-  [MinVersion=16] OnInstallationStarted@14();
+  // Notifies that Play Store installation has been started. |package_name|
+  // specifies installation package
+  [MinVersion=16] OnInstallationStarted@14(
+      [MinVersion=17] string? package_name@0);

[dcheng, 2017/01/05 06:54:05]

How come the package name is optional here?

[khmel, 2017/01/05 15:24:29]

My intention here to support versions < 17. If I declare it as required  , as I
understand, in case Android side version is 16, it would be impossible to make
this call.

 
-  // Notifies that Play Store installation is finished.
-  [MinVersion=16] OnInstallationFinished@15();
+  // Notifies that Play Store installation is finished. |result| contains 
+  // details of installation result.
+  [MinVersion=16] OnInstallationFinished@15(
+      [MinVersion=17] InstallationResult? result@1);
 
   // Notifies that task requested orientation lock.
   [MinVersion=12] OnTaskOrientationLockRequested@12(int32 task_id,