Add pathway to auto-migrate users when Chrome launches

chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java

Index: chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java b/chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java
index 2e655d14b8b951db14fcc34a7fd4667a6bcacff7..cf5469ab42e483c3d222248f37f58dc19c884a26 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/preferences/MainPreferences.java
@@ -11,7 +11,10 @@ import android.preference.Preference;
 import android.preference.Preference.OnPreferenceClickListener;
 import android.preference.PreferenceFragment;
 
+import org.chromium.base.ApplicationStatus;
+import org.chromium.base.CommandLine;
 import org.chromium.chrome.R;
+import org.chromium.chrome.browser.ChromeSwitches;
 import org.chromium.chrome.browser.PasswordUIView;
 import org.chromium.chrome.browser.autofill.PersonalDataManager;
 import org.chromium.chrome.browser.net.spdyproxy.DataReductionProxySettings;
@@ -103,8 +106,14 @@ public class MainPreferences extends PreferenceFragment implements SignInStateOb
         });
         mSignInPreference.setEnabled(true);
 
+        // TODO(dfalcantara): Delete this preference entirely.  https://crbug.com/582539
         Preference documentMode = findPreference(PREF_DOCUMENT_MODE);
-        if (FeatureUtilities.isDocumentModeEligible(getActivity())) {
+        boolean showDocumentToggle = FeatureUtilities.isDocumentModeEligible(getActivity());
+        if (CommandLine.getInstance().hasSwitch(ChromeSwitches.ENABLE_FORCED_MIGRATION)) {
+            showDocumentToggle &=
+                    FeatureUtilities.isDocumentMode(ApplicationStatus.getApplicationContext());
+        }
+        if (showDocumentToggle) {
             setOnOffSummary(documentMode,
                     !DocumentModeManager.getInstance(getActivity()).isOptedOutOfDocumentMode());
         } else {