Make Android back work correctly in Elderberry.

chrome/android/java/src/org/chromium/chrome/browser/document/ChromeLauncherActivity.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.document;
 
 import android.annotation.SuppressLint;
 import android.app.Activity;
 import android.app.Notification;
 import android.app.PendingIntent;
@@ skipping 330 matching lines @@
         // For Elderberry flavored Herbs that are to be launched in a separate task, add a random
         // UUID to try and prevent Android from refocusing/clobbering items that share the same
         // base intent.  If we do support refocusing of existing Herbs, we need to do it on the
         // current URL and not the URL that it was triggered with.
         if (TextUtils.equals(
                 FeatureUtilities.getHerbFlavor(), ChromeSwitches.HERB_FLAVOR_ELDERBERRY)
                 && (newIntent.getFlags() & Intent.FLAG_ACTIVITY_NEW_TASK) != 0
                         || (newIntent.getFlags() & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0) {
             newIntent.setClassName(context, SeparateTaskCustomTabActivity.class.getName());
 
+            // Pre-L, the exclude from recents flag on the launcher does not apply to the launched
+            // separate task activity (and provides the desired user behavior).  On L, the flag
+            // needs to be cleared otherwise it is hidden immediately upon exiting.
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+                newIntent.setFlags(
+                        newIntent.getFlags() & ~Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
+            }
             String url = IntentHandler.getUrlFromIntent(newIntent);
             assert url != null;
 
             newIntent.setData(new Uri.Builder().scheme(UrlConstants.CUSTOM_TAB_SCHEME)
                     .authority(UUID.randomUUID().toString())
                     .query(url).build());
         }
 
         Bundle herbActionButtonBundle = new Bundle();
 
@@ skipping 143 matching lines @@
         IntentHandler.ExternalAppId source =
                 IntentHandler.determineExternalIntentSource(getPackageName(), intent);
         if (intent.getPackage() == null && source != IntentHandler.ExternalAppId.CHROME) {
             int flagsOfInterest = Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
             int maskedFlags = intent.getFlags() & flagsOfInterest;
             sIntentFlagsHistogram.record(maskedFlags);
         }
         MediaNotificationUma.recordClickSource(intent);
     }
 }