navigation: Retain a screenshot of the page before it unloads

content/browser/web_contents/navigation_entry_impl.h

Index: content/browser/web_contents/navigation_entry_impl.h
diff --git a/content/browser/web_contents/navigation_entry_impl.h b/content/browser/web_contents/navigation_entry_impl.h
index ba13c147fd44b14eed9b8cae8fc866075ec15c24..39ae523b8ea2abe86c0c706a2ea5a62607bb196d 100644
--- a/content/browser/web_contents/navigation_entry_impl.h
+++ b/content/browser/web_contents/navigation_entry_impl.h
@@ -167,6 +167,11 @@ class CONTENT_EXPORT NavigationEntryImpl
     should_replace_entry_ = should_replace_entry;
   }
 
+  void SetScreenshotPNGData(const std::vector<unsigned char>& png_data);
+  const scoped_refptr<base::RefCountedBytes> screenshot() const {
+    return screenshot_;
+  }
+
  private:
   // WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING
   // Session/Tab restore save portions of this class so that it can be recreated
@@ -203,6 +208,10 @@ class CONTENT_EXPORT NavigationEntryImpl
   // copied with compiler provided copy constructor.
   scoped_refptr<const base::RefCountedMemory> browser_initiated_post_data_;
 
+  // This is also a transient memeber (i.e. is not persisted with session
+  // restore).

[Charlie Reis, 2012/12/21 20:01:08]

What would happen then if you restored a session and tried to swipe back?

[sadrul, 2012/12/21 20:13:24]

In the absence of a screenshot (after a restore, or if taking the screenshot
failed for some reason), a placeholder image (currently all-gray) is displayed
instead.

+  scoped_refptr<base::RefCountedBytes> screenshot_;
+
   // This member is not persisted with session restore.
   std::string extra_headers_;