Enabling overscroll/GestureNav for Athena.

BUG=401604

Review URL: https://codereview.chromium.org/524623004

Cr-Commit-Position: refs/heads/master@{#292918}

athena/content/web_activity.cc

@@ -8,11 +8,13 @@
 #include "athena/activity/public/activity_manager.h"
 #include "athena/input/public/accelerator_manager.h"
 #include "base/bind.h"
+#include "base/command_line.h"
 #include "base/strings/utf_string_conversions.h"
 #include "content/public/browser/native_web_keyboard_event.h"
 #include "content/public/browser/navigation_controller.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_delegate.h"
+#include "content/public/common/content_switches.h"
 #include "ui/aura/window.h"
 #include "ui/compositor/closure_animation_observer.h"
 #include "ui/compositor/scoped_layer_animation_settings.h"
@@ -231,6 +233,12 @@ class AthenaWebView : public views::WebView {
     return NULL;
   }
 
+  virtual bool CanOverscrollContent() const OVERRIDE {
+    const std::string value = CommandLine::ForCurrentProcess()->
+        GetSwitchValueASCII(switches::kOverscrollHistoryNavigation);
+    return value != "0";
+  }
+
   virtual void AddNewContents(content::WebContents* source,
                               content::WebContents* new_contents,
                               WindowOpenDisposition disposition,