implement user agent override option.

ios/web/public/navigation_manager.h

Index: ios/web/public/navigation_manager.h
diff --git a/ios/web/public/navigation_manager.h b/ios/web/public/navigation_manager.h
index 876cee6d046edfbfb4518308d233dd4bf9c4b8d0..57715435180872c206a24e94bd637d382b5210f2 100644
--- a/ios/web/public/navigation_manager.h
+++ b/ios/web/public/navigation_manager.h
@@ -30,6 +30,19 @@ class WebState;
 // exactly one NavigationManager.
 class NavigationManager {
  public:
+  // User agent override option used in LoadURLParams.
+  enum class UserAgentOverrideOption : short {
+    // Inherit the user agent type from the last committed non-app specific item

[Eugene But (OOO till 7-30), 2017/03/30 19:21:24]

What if there are no committed items?

[liaoyuke, 2017/03/30 19:56:25]

Done. 

+    // in the NavigationManager.
+    INHERIT = 0,
+
+    // Use the mobile user agent.
+    MOBILE,
+
+    // Use the desktop user agent.
+    DESKTOP

[Eugene But (OOO till 7-30), 2017/03/30 19:21:24]

Optional nit: it's a good practice to have comma after last item. Sp the future
changes will not touch this line.

[liaoyuke, 2017/03/30 19:56:25]

Done.

+  };
+
   // Parameters for URL loading. Most parameters are optional, and can be left
   // at the default values set by the constructor.
   struct WebLoadParams {
@@ -43,6 +56,9 @@ class NavigationManager {
     // The transition type for the load. Defaults to PAGE_TRANSITION_LINK.
     ui::PageTransition transition_type;
 
+    // The user agent override option for the load. Defualts to INHERIT.
+    UserAgentOverrideOption user_agent_override_option;
+
     // True for renderer-initiated navigations. This is
     // important for tracking whether to display pending URLs.
     bool is_renderer_initiated;
@@ -179,12 +195,6 @@ class NavigationManager {
   // - There is no pending history navigation.
   // - There is no transient NavigationItem.
   virtual bool CanPruneAllButLastCommittedItem() const = 0;
-
-  // Forces the pending item to be loaded using desktop user agent. Note that
-  // the pending item may or may not already exist.
-  // TODO(crbug.com/692303): Remove this when overriding the user agent doesn't
-  // create a new NavigationItem.
-  virtual void OverrideDesktopUserAgentForNextPendingItem() = 0;
 };
 
 }  // namespace web