[wip] Introduce SiteInstanceKeepAlive.

content/browser/frame_host/navigation_entry_screenshot_manager.h

Index: content/browser/frame_host/navigation_entry_screenshot_manager.h
diff --git a/content/browser/frame_host/navigation_entry_screenshot_manager.h b/content/browser/frame_host/navigation_entry_screenshot_manager.h
index 072d6fd4a7fe19a1111acdb6b2e5c6c470f92285..4ec87815919111515b9c48bad39e2a619ba32f5b 100644
--- a/content/browser/frame_host/navigation_entry_screenshot_manager.h
+++ b/content/browser/frame_host/navigation_entry_screenshot_manager.h
@@ -6,9 +6,12 @@
 #define CONTENT_BROWSER_FRAME_HOST_NAVIGATION_ENTRY_SCREENSHOT_MANAGER_H_
 
 #include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
 #include "content/common/content_export.h"
+#include "content/public/browser/web_contents_observer.h"
 
 class SkBitmap;
 
@@ -18,12 +21,14 @@ class NavigationControllerImpl;
 class NavigationEntryImpl;
 class RenderViewHost;
 class ScreenshotData;
+class SiteInstanceKeepAlive;
 
 // NavigationEntryScreenshotManager takes care of taking image-captures for the
 // current navigation entry of a NavigationControllerImpl, and managing these
 // captured images. These image-captures are used for history navigation using
 // overscroll gestures.
-class CONTENT_EXPORT NavigationEntryScreenshotManager {
+class CONTENT_EXPORT NavigationEntryScreenshotManager
+    : public WebContentsObserver {
  public:
   explicit NavigationEntryScreenshotManager(
       NavigationControllerImpl* controller);
@@ -50,6 +55,7 @@ class CONTENT_EXPORT NavigationEntryScreenshotManager {
   // The callback invoked when taking the screenshot of the page is complete.
   // This sets the screenshot on the navigation entry.
   void OnScreenshotTaken(int unique_id,
+                         SiteInstanceKeepAlive* keep_alive,
                          bool success,
                          const SkBitmap& bitmap);
 
@@ -71,9 +77,16 @@ class CONTENT_EXPORT NavigationEntryScreenshotManager {
   // consumption is within a certain limit.
   void PurgeScreenshotsIfNecessary();
 
+  // WebContentsObserver:
+  virtual void WebContentsDestroyed(WebContents* contents) OVERRIDE;
+
   // The navigation controller that owns this screenshot-manager.
   NavigationControllerImpl* owner_;
 
+  // Keeps track of (and owns) all the SiteInstanceKeepAlive objects created for
+  // screenshot requests.
+  ScopedVector<SiteInstanceKeepAlive> keep_alives_;
+
   // Taking a screenshot and encoding them can be async. So use a weakptr for
   // the callback to make sure that the screenshot/encoding completion callback
   // does not trigger on a destroyed NavigationEntryScreenshotManager.