Purge once random minutes(between 30min and 60min) after backgrounded.

chrome/browser/memory/tab_manager_web_contents_data.h

Index: chrome/browser/memory/tab_manager_web_contents_data.h
diff --git a/chrome/browser/memory/tab_manager_web_contents_data.h b/chrome/browser/memory/tab_manager_web_contents_data.h
index 8f68c1239dec7c76096869da0df22681c15ff81c..796ca3d744a25ee41a3dcc42d3369f8bcb287161 100644
--- a/chrome/browser/memory/tab_manager_web_contents_data.h
+++ b/chrome/browser/memory/tab_manager_web_contents_data.h
@@ -79,25 +79,23 @@ class TabManager::WebContentsData
   // Sets/clears the auto-discardable state of the tab.
   void SetAutoDiscardableState(bool state);
 
-  // Sets the current purge-and-suspend state, and update the timestamp,
-  // i.e. the last purge-and-suspend modified time.
-  // TODO(tasak): remove this after PurgeAndSuspend code is moved into
+  // Sets the current purge state.
+  // TODO(tasak): remove this after the logic is moved into
   // MemoryCoordinator.
-  void SetPurgeAndSuspendState(TabManager::PurgeAndSuspendState state);
+  void set_is_purged(bool state) { is_purged_ = state; }
 
-  // Returns the last purge-and-suspend modified time.
-  // TODO(tasak): remove this after PurgeAndSuspend code is moved into
+  // Returns the current state of purge.
+  // TODO(tasak): remove this after the logic is moved into
   // MemoryCoordinator.
-  base::TimeTicks LastPurgeAndSuspendModifiedTime() const;
+  bool is_purged() const { return is_purged_; }
 
-  // Sets the timestamp of the last modified time the purge-and-suspend state
-  // of the tab was changed for testing.
-  void SetLastPurgeAndSuspendModifiedTimeForTesting(base::TimeTicks timestamp);
+  // Sets the time to purge after the tab is backgrounded.
+  void set_time_to_purge(const base::TimeDelta& time_to_purge) {
+    time_to_purge_ = time_to_purge;
+  }
 
-  // Returns the current state of purge-and-suspend.
-  // TODO(tasak): remove this after PurgeAndSuspend code is moved into
-  // MemoryCoordinator.
-  TabManager::PurgeAndSuspendState GetPurgeAndSuspendState() const;
+  // Returns the time to first purge after the tab is backgrounded.
+  base::TimeDelta time_to_purge() const { return time_to_purge_; }
 
  private:
   // Needed to access tab_data_.
@@ -141,11 +139,20 @@ class TabManager::WebContentsData
   // this test clock. Otherwise it returns the system clock's value.
   base::TickClock* test_tick_clock_;
 
-  // The last time purge-and-suspend state was modified.
-  base::TimeTicks last_purge_and_suspend_modified_time_;
-
-  // The current state of purge-and-suspend.
-  PurgeAndSuspendState purge_and_suspend_state_;
+  // The time to purge after the tab is backgrounded.
+  base::TimeDelta time_to_purge_;
+
+  // True if the tab has been purged since it was last active, and should
+  // therefore not be purged again.
+  // The rest of the detail of the algorithm doesn't belong here; the only
+  // detail to keep here would be:

[Wez, 2017/03/03 21:06:51]

I would actually remove these transition descriptions entirely, and document
them alongside the code that implements the logic e.g. at ShouldPurgeNow()

[tasak, 2017/03/06 10:30:19]

I moved the document to TabManager::ActiveTabChanged, and
TabManager::PurgeBackgroundedTabsIfNeeded.

+  // false => true
+  // - A tab is kept inactive and background for more than time-to-purge time.
+  // true => false
+  // - When ActiveTabChaged, the newly activated tab's state will be false.
+  // When a discarded tab is restored, we set |is_purged_=true|,
+  // since purging the newly-created renderer doesn't make sense.
+  bool is_purged_;
 
   DISALLOW_COPY_AND_ASSIGN(WebContentsData);
 };