Add pathway to auto-migrate users when Chrome launches

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java

Index: chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
index 89936ef92c474a4c2dc7034bfa1a1e3357aaa8f5..a0861764b772eadc4f2ebc522968e16251412455 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
@@ -12,6 +12,7 @@ import android.os.Build;
 import android.util.Pair;
 
 import org.chromium.base.ApplicationStatus;
+import org.chromium.base.CommandLine;
 import org.chromium.base.FileUtils;
 import org.chromium.base.Log;
 import org.chromium.base.ObserverList;
@@ -19,6 +20,7 @@ import org.chromium.base.StreamUtil;
 import org.chromium.base.ThreadUtils;
 import org.chromium.base.VisibleForTesting;
 import org.chromium.chrome.browser.ChromeApplication;
+import org.chromium.chrome.browser.ChromeSwitches;
 import org.chromium.chrome.browser.TabState;
 import org.chromium.chrome.browser.document.DocumentActivity;
 import org.chromium.chrome.browser.document.DocumentUtils;
@@ -105,7 +107,7 @@ public class DocumentModeAssassin {
     static final int STAGE_CHANGE_SETTINGS_STARTED = 6;
     static final int STAGE_CHANGE_SETTINGS_DONE = 7;
     static final int STAGE_DELETION_STARTED = 8;
-    static final int STAGE_DONE = 9;
+    public static final int STAGE_DONE = 9;
 
     private static final String TAG = "DocumentModeAssassin";
 
@@ -137,7 +139,8 @@ public class DocumentModeAssassin {
 
     /** Returns whether or not a migration to tabbed mode from document mode is necessary. */
     public static boolean isMigrationNecessary() {
-        return FeatureUtilities.isDocumentMode(ApplicationStatus.getApplicationContext());
+        return CommandLine.getInstance().hasSwitch(ChromeSwitches.ENABLE_FORCED_MIGRATION)
+                && FeatureUtilities.isDocumentMode(ApplicationStatus.getApplicationContext());
     }
 
     /** Migrates the user from document mode to tabbed mode if necessary. */
@@ -471,6 +474,7 @@ public class DocumentModeAssassin {
      */
     @VisibleForTesting
     public int getStage() {
+        ThreadUtils.assertOnUiThread();
         return mStage;
     }