tab_discarder: check nullptr for web_contents_data

Based on the bug report, the GetTabFeatures() crashes on a line when
accessing it's own member variables which indicates the pointer maybe
empty.

Based on recent change in
https://chromium-review.googlesource.com/c/chromium/src/+/1816004

It's very likely that web_contents_data becomes nullptr in some cases.

Bug: 1017222
Change-Id: Iec58d533af478cc37f96cbedc263884e7e34bf1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884407Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711467}

chrome/browser/resource_coordinator/tab_activity_watcher.cc

@@ -568,15 +568,22 @@ void TabActivityWatcher::LogAndMaybeSortLifecycleUnitWithTabRanker(
     // TabManagerDelegateTest::KillMultipleProcesses.
     if (!lifecycle_unit_external) {
       tab_features[lifecycle_unit->GetID()] = base::nullopt;
-    } else {
-      WebContentsData* web_contents_data = WebContentsData::FromWebContents(
-          lifecycle_unit_external->GetWebContents());
-      DCHECK(web_contents_data);
-      const base::Optional<TabFeatures> tab =
-          web_contents_data->GetTabFeatures();
-      tab_features[lifecycle_unit->GetID()] = tab;
-      web_contents_data->LogCurrentTabFeatures(tab);
+      continue;
     }
+    WebContentsData* web_contents_data = WebContentsData::FromWebContents(
+        lifecycle_unit_external->GetWebContents());
+
+    // The web_contents_data can be nullptr in some cases.
+    // TODO(crbug.com/1019482): move the creation of WebContentsData to
+    // TabHelpers::AttachTabHelpers.
+    if (!web_contents_data) {
+      tab_features[lifecycle_unit->GetID()] = base::nullopt;
+      continue;
+    }
+
+    const base::Optional<TabFeatures> tab = web_contents_data->GetTabFeatures();
+    tab_features[lifecycle_unit->GetID()] = tab;
+    web_contents_data->LogCurrentTabFeatures(tab);
   }
 
   // Directly return if TabRanker is not enabled.