[Favicon] Add new fallback icon rendering flow.

chrome/browser/favicon/favicon_service.h

Index: chrome/browser/favicon/favicon_service.h
diff --git a/chrome/browser/favicon/favicon_service.h b/chrome/browser/favicon/favicon_service.h
index 06c1a66ec29bf9c5095da64abffda5248d4ffa52..293f443fbe92fd25267423aa28556b1ddd934ba9 100644
--- a/chrome/browser/favicon/favicon_service.h
+++ b/chrome/browser/favicon/favicon_service.h
@@ -5,6 +5,7 @@
 #ifndef CHROME_BROWSER_FAVICON_FAVICON_SERVICE_H_
 #define CHROME_BROWSER_FAVICON_FAVICON_SERVICE_H_
 
+#include <memory>
 #include <vector>
 
 #include "base/callback.h"
@@ -21,6 +22,11 @@ class HistoryService;
 struct ImportedFaviconUsage;
 class Profile;
 
+namespace favicon_base {
+class FallbackIconService;
+struct FallbackIconSpecs;
+}  // namespace favicon_base
+
 // The favicon service provides methods to access favicons. It calls the history
 // backend behind the scenes. The callbacks are run asynchronously, even in the
 // case of an error.
@@ -79,6 +85,18 @@ class FaviconService : public KeyedService {
       const favicon_base::FaviconResultsCallback& callback,
       base::CancelableTaskTracker* tracker);
 
+  // Generates a fallback favicon for |icon_url| with size |size_in_pixel|,
+  // using auxiliary rendering parameters in |params|. |size_in_pixel| = 0
+  // specifies maximum fallback favicon size. |specs| does not need to live
+  // beyond the function call.

[pkotwicz, 2015/01/19 04:32:02]

My hope is that this function will never be needed. Ideally, the only way to get
a fallback icon would be by requesting a normal favicon for icon URL if:
- a favicon for |icon_url| was not stored at the desired size in the favicon
database.

For now FallbackFaviconSource (not FaviconSource) can call directly into
FallbackIconService

[huangs, 2015/01/20 22:20:41]

Simplified, and named everything "FallbackIcon" or "fallback-icon".

+  base::CancelableTaskTracker::TaskId GetRawFallbackFaviconImage(
+      const GURL& icon_url,
+      favicon_base::IconType icon_type,
+      int desired_size_in_pixel,
+      const favicon_base::FallbackIconSpecs& specs,
+      const favicon_base::FaviconRawBitmapCallback& callback,
+      base::CancelableTaskTracker* tracker);
+
   //////////////////////////////////////////////////////////////////////////////
   // Methods to request favicon bitmaps from the history backend for |page_url|.
   // |page_url| is the web page the favicon is associated with.
@@ -214,6 +232,7 @@ class FaviconService : public KeyedService {
   typedef uint32 MissingFaviconURLHash;
   base::hash_set<MissingFaviconURLHash> missing_favicon_urls_;
   HistoryService* history_service_;
+  std::unique_ptr<favicon_base::FallbackIconService> fallback_icon_service_;
   Profile* profile_;
   FaviconClient* favicon_client_;