Resume a backgrounded renderer that was purged and suspended

chrome/browser/memory/tab_manager.h

Index: chrome/browser/memory/tab_manager.h
diff --git a/chrome/browser/memory/tab_manager.h b/chrome/browser/memory/tab_manager.h
index 9dc86df35fb7da85d33e17f02a3eae04f168d72e..f424521a4b9908a5f908ff3257b74f8f53d42eb7 100644
--- a/chrome/browser/memory/tab_manager.h
+++ b/chrome/browser/memory/tab_manager.h
@@ -174,8 +174,9 @@ class TabManager : public TabStripModelObserver {
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ReloadDiscardedTabContextMenu);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, TabManagerBasics);
 
-  // This is needed so WebContentsData can call OnDiscardedStateChange.
-  friend class WebContensData;
+  // This is needed so WebContentsData can call OnDiscardedStateChange, and
+  // can use PurgeAndSuspendState.
+  friend class WebContentsData;
 
   // Called by WebContentsData whenever the discard state of a WebContents
   // changes, so that observers can be informed.
@@ -247,6 +248,43 @@ class TabManager : public TabStripModelObserver {
   // that need to be run periodically (see comment in implementation).
   void UpdateTimerCallback();
 
+  // Returns WebContents whose contents id matches the given tab-contents-id.
+  content::WebContents* GetWebContentsByContentsId(int64_t tab_contents_id);
+
+  // Initially PurgeAndSuspendState is RUNNING.
+  // RUNNING => SUSPENDED
+  // - Some tab has been backgrounded for more than purge-and-suspend-time
+  //   seconds.
+  // SUSPENDED => RESUMED
+  // - A suspended tab is still suspended (i.e. last active time < last
+  //   purge-and-suspend modified time), and
+  // - kMaxTimeRendererAllowedToBeSuspendedBeforeResume time passes since
+  //   since the tab was suspended.
+  // RESUMED => SUSPENDED
+  // - A resumed tab is still backgrounded (i.e. last active time < last
+  //   purge-and-suspend modified time), and
+  // - kSuspendedRendererLengthOfResumption time passes since the tab was
+  //   resumed.
+  // SUSPENDED, RESUMED => SUSPENDED
+  // - Since it is difficult to capture OnProcessBackgrounded or
+  //   OnProcessForegrounded in TabManager, compare last active time with
+  //   last purge-and-suspend modified time.
+  // - if last active time > last purge-and-suspend modified time,
+  //   the tab was foregrounded and is not suspended.
+  //   So we treat the state as RUNNING.
+  enum PurgeAndSuspendState {
+    RUNNING,
+    RESUMED,
+    SUSPENDED,
+  };
+  // Determines whether to update purge-and-suspend state and returns the
+  // next state.
+  bool ShouldUpdatePurgeAndSuspendState(
+      const base::TimeTicks& current_time,
+      const TabStats& tab,
+      const base::TimeDelta& purge_and_suspend_threshold,
+      PurgeAndSuspendState* next_state);
+
   // Purges and suspends renderers in backgrounded tabs.
   void PurgeAndSuspendBackgroundedTabs();