Fix window.open crashes on custom tabs

chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java

Index: chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java b/chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java
index 5604a83b7b1f9f68c69863fb988e9c393b33bc6a..4f7869e8622462011d760637faa087bb43ae4f08 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tab/ChromeTab.java
@@ -369,8 +369,8 @@ public class ChromeTab extends Tab {
 
             // TODO(dfalcantara): Re-remove this once crbug.com/508366 is fixed.
             TabCreator tabCreator = mActivity.getTabCreator(isIncognito());
-            assert tabCreator != null;
-            if (tabCreator.createsTabsAsynchronously()) {
+
+            if (tabCreator != null && tabCreator.createsTabsAsynchronously()) {
                 DocumentWebContentsDelegate.getInstance().attachDelegate(newWebContents);
             }
         }