[Thumbnails DB] Allow setting last_requested time when accessing favicons.

components/favicon/core/favicon_service.h

Index: components/favicon/core/favicon_service.h
diff --git a/components/favicon/core/favicon_service.h b/components/favicon/core/favicon_service.h
index d36180093497dc83e2031acae57945ebdfbb56be..de05b365875230631e00c4b6b3456df09c4a24bd 100644
--- a/components/favicon/core/favicon_service.h
+++ b/components/favicon/core/favicon_service.h
@@ -139,6 +139,15 @@ class FaviconService : public KeyedService {
   // Marks all types of favicon for the page as being out of date.
   virtual void SetFaviconOutOfDateForPage(const GURL& page_url) = 0;
 
+  // Mark that the on-demand favicon at |icon_url| was requested now. This
+  // postpones the automatic eviction of the favicon from the database. Not all
+  // calls end up in a write into the DB:
+  // - it is no-op if the bitmaps are not stored using SetOnDemandFavicons();
+  // - the updates of the last requested time have limited frequency for each
+  //   particular favicon (e.g. once per week). This limits the overhead of
+  //   cache management for on-demand favicons.
+  virtual void TouchOnDemandFavicon(const GURL& icon_url) = 0;
+
   // Allows the importer to set many favicons for many pages at once. The pages
   // must exist, any favicon sets for unknown pages will be discarded. Existing
   // favicons will not be overwritten.
@@ -171,19 +180,29 @@ class FaviconService : public KeyedService {
                            favicon_base::IconType icon_type,
                            const gfx::Image& image) = 0;
 
-  // Same as SetFavicons() with three differences:
+  // Same as SetFavicons with three differences:
   // 1) It will be a no-op if there is an existing cached favicon for *any* type
   //    for |page_url|.
   // 2) If |icon_url| is known to the database, |bitmaps| will be ignored (i.e.
   //    the icon won't be overwritten) but the mappings from |page_url| to
   //    |icon_url| will be stored (conditioned to point 1 above).
-  // 3) If |icon_url| is stored, it will be marked as expired.
+  // 3) If |icon_url| is stored, it will be marked as "on-demand".
+  //
+  // On-demand favicons are those that are fetched without visiting their page.
+  // For this reason, their life-time cannot be bound to the life-time of the
+  // corresponding visit in history.
+  // - These bitmaps are evicted from the database based on the last time they
+  //   get requested. The last requested time is initially set to Now() and is
+  //   further updated by calling TouchOnDemandFavicon().
+  // - Furthermore, on-demand bitmaps are immediately marked as expired. Hence,
+  //   they are always replaced by standard favicons whenever their page gets
+  //   visited.
   // The callback will receive whether the write actually happened.
-  virtual void SetLastResortFavicons(const GURL& page_url,
-                                     const GURL& icon_url,
-                                     favicon_base::IconType icon_type,
-                                     const gfx::Image& image,
-                                     base::Callback<void(bool)> callback) = 0;
+  virtual void SetOnDemandFavicons(const GURL& page_url,
+                                   const GURL& icon_url,
+                                   favicon_base::IconType icon_type,
+                                   const gfx::Image& image,
+                                   base::Callback<void(bool)> callback) = 0;
 
   // Avoid repeated requests to download missing favicon.
   virtual void UnableToDownloadFavicon(const GURL& icon_url) = 0;