Improve syncing between dom distiller and the print preview request

components/dom_distiller/core/dom_distiller_service.h

Index: components/dom_distiller/core/dom_distiller_service.h
diff --git a/components/dom_distiller/core/dom_distiller_service.h b/components/dom_distiller/core/dom_distiller_service.h
index 5d2558d1142b68467f6434f8b538663a16cd7762..3d9f7f9d079217453435abb92067171657c04089 100644
--- a/components/dom_distiller/core/dom_distiller_service.h
+++ b/components/dom_distiller/core/dom_distiller_service.h
@@ -92,6 +92,38 @@ class DomDistillerServiceInterface {
       scoped_ptr<DistillerPage> distiller_page,
       const GURL& url) = 0;
 
+  // Adds |delegate| to the provided |url| task tracker.
+  // If the given |url| has no task tracker (i.e. has no current
+  // distillation request underway), this will return nullptr - therefore
+  // this should only be called after previously requesting a distillation.
+
+  // If the inclusion of |delegate| on the |url| task tracker was
+  // successful, returns a |ViewerHandle| from the task tracker. This
+  // |ViewerHandle| can be held by client code as long as it wants to
+  // receive callbacks as a |ViewRequestDelegate|.
+
+  // Upon successful inclusion (i.e. a |ViewerHandle| was returned),
+  // the following holds:
+  //
+  // 1) client code will get all future OnArticleUpdated() callbacks for
+  // as long as the distillation process is running.
+  //
+  // 2) client code will get at least the OnArticleFinished()
+  // callback, considering:
+  // 2.a) If the article distillation is not finished yet, the
+  // callback will trigger at the same time as when the original
+  // distillation request gets it.
+  // 2.b) Client code will immediately get this callback after adding
+  // the viewer. If the original distillation request has already
+  // gotten this callback, it has also previously posted it
+  // to a valid standing task tracker.
+
+  // If the inclusion was not successful (i.e. this function
+  // returned nullptr), client code will not get any callbacks.
+  virtual scoped_ptr<ViewerHandle> AddViewRequestDelegate(
+      ViewRequestDelegate* delegate,
+      const GURL& url) = 0;
+
   // Creates a default DistillerPage.
   virtual scoped_ptr<DistillerPage> CreateDefaultDistillerPage(
       const gfx::Size& render_view_size) = 0;
@@ -137,6 +169,8 @@ class DomDistillerService : public DomDistillerServiceInterface {
   scoped_ptr<ViewerHandle> ViewUrl(ViewRequestDelegate* delegate,
                                    scoped_ptr<DistillerPage> distiller_page,
                                    const GURL& url) override;
+  scoped_ptr<ViewerHandle> AddViewRequestDelegate(ViewRequestDelegate* delegate,
+                                                  const GURL& url) override;
   scoped_ptr<DistillerPage> CreateDefaultDistillerPage(
       const gfx::Size& render_view_size) override;
   scoped_ptr<DistillerPage> CreateDefaultDistillerPageWithHandle(