Make DocumentTabModelImpl fire didSelectTab

DocumentTabModelImpl now fires didSelectTab to its observers. This is done in
setLastShownId() instead of setIndex() for a few reasons:

* setIndex() has the side effect of bringing Tabs forward, which you may not
  want.

* setLastShownId() hopefully more faithfully tracks what Tab was last shown in
  the foreground, and is called by DocumentActivity.onResume().  This might
  still be incorrect (e.g. when opening a Tab in the background), but this is
  one of those weird cases where Android has two Activities in the foreground
  simultaneously.

* It's really difficult to actually indicate what brings a Tab back to the
  foreground.  Slap a TODO on there to actually fix it when an Observer actually
  needs to know the type.

BUG=443772

Review URL: https://codereview.chromium.org/1039513003

Cr-Commit-Position: refs/heads/master@{#322519}

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/document/DocumentTabModel.java

@@ -195,8 +195,9 @@ public interface DocumentTabModel extends TabModel {
     /**
      * Records the ID of the last shown Tab.
      * @param id ID of the last shown Tab.
+     * @return Whether or not the ID had to be updated.
      */
-    void setLastShownId(int id);
+    boolean setLastShownId(int id);
 
     /**
      * Called to begin loading tab state from disk. It will load the prioritized tab first

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/document/DocumentTabModelImpl.java

@@ -223,7 +223,10 @@ public class DocumentTabModelImpl extends TabModelJniBridge implements DocumentT
     }
 
     @Override
-    public void setLastShownId(int id) {
+    public boolean setLastShownId(int id) {
+        if (mLastShownTabId == id) return false;
+
+        int previousTabId = mLastShownTabId;
         mLastShownTabId = id;
 
         String prefName =
@@ -232,6 +235,15 @@ public class DocumentTabModelImpl extends TabModelJniBridge implements DocumentT
         SharedPreferences.Editor editor = prefs.edit();
         editor.putInt(prefName, id);
         editor.apply();
+
+        // TODO(dfalcantara): Figure out how to fire the correct type of TabSelectionType, which is
+        //                    quite hard to do in Document-mode from where we call this.
+        for (TabModelObserver obs : mObservers) {
+            obs.didSelectTab(
+                    TabModelUtils.getCurrentTab(this), TabSelectionType.FROM_USER, previousTabId);
+        }
+
+        return true;
     }
 
     @Override

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/document/OffTheRecordDocumentTabModel.java

@@ -142,9 +142,9 @@ public class OffTheRecordDocumentTabModel extends OffTheRecordTabModel implement
     }
 
     @Override
-    public void setLastShownId(int id) {
+    public boolean setLastShownId(int id) {
         ensureTabModelImpl();
-        getDelegateDocumentTabModel().setLastShownId(id);
+        return getDelegateDocumentTabModel().setLastShownId(id);
     }
 
     @Override