[Document mode] Don't ignore tabs that don't have an initial URL

Tasks without initial URLs weren't being counted as legitimate Tabs,
preventing them from being restored properly.

BUG=481109

Review URL: https://codereview.chromium.org/1109543003

Cr-Commit-Position: refs/heads/master@{#326944}

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

@@ -91,8 +91,9 @@ public class ActivityDelegate {
             if (!isValidActivity(isIncognito, intent)) continue;
 
             int tabId = getTabIdFromIntent(intent);
+            if (tabId == Tab.INVALID_TAB_ID) continue;
+
             String initialUrl = getInitialUrlForDocument(intent);
-            if (tabId == Tab.INVALID_TAB_ID || initialUrl == null) continue;
             entries.add(new Entry(tabId, initialUrl));
         }
         return entries;