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);

[Charlie Reis, 2012/12/21 23:22:07]

We might be prematurely optimizing with PNG (especially putting it in the
function name).  Your suggestion of capping the amount of space we use might be
sufficient to avoid the encoding/decoding costs, or we might find that WebP is a
better option.

[sadrul, 2012/12/22 00:07:10]

I didn't have optimization in mind when I used the PNG data here :)

We get an SkBitmap from the screenshot callback, and we need to store the data
in a way that can be copied (since NavigationEntryImpl is copy-able) without
much difficulty, and also displayed by the UI code. PNG seemed like a good
choice for this.

+  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

[Charlie Reis, 2012/12/21 23:22:07]

typo: memeber

Also, please explain when this screenshot is taken, why it is needed, and why
it's ok for it to be missing.

[sadrul, 2012/12/22 00:07:10]

Done.

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