[Start] Add a null check of FeedStream in StartSurfaceMediator.

The crash was caused by registering a listener to a null feedStream.

Bug: 1127854
Change-Id: Ic040cf6620718e72b7f8a3faac115f7fb8799f34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407093Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807143}

chrome/android/features/start_surface/internal/java/src/org/chromium/chrome/features/start_surface/StartSurfaceMediator.java

@@ -335,15 +335,16 @@ class StartSurfaceMediator
                     mPropertyModel.get(FEED_SURFACE_COORDINATOR);
             if (feedSurfaceCoordinator != null) {
                 Stream feedStream = feedSurfaceCoordinator.getStream();
-                assert feedStream != null;
-                feedStream.addOnContentChangedListener(() -> {
-                    int firstCardDensity = feedStream.getFirstCardDensity();
-                    if (firstCardDensity != Stream.FeedFirstCardDensity.UNKNOWN) {
-                        StartSurfaceConfiguration.setFeedPlaceholderDense(
-                                feedStream.getFirstCardDensity()
-                                == Stream.FeedFirstCardDensity.DENSE);
-                    }
-                });
+                if (feedStream != null) {
+                    feedStream.addOnContentChangedListener(() -> {
+                        int firstCardDensity = feedStream.getFirstCardDensity();
+                        if (firstCardDensity != Stream.FeedFirstCardDensity.UNKNOWN) {
+                            StartSurfaceConfiguration.setFeedPlaceholderDense(
+                                    feedStream.getFirstCardDensity()
+                                    == Stream.FeedFirstCardDensity.DENSE);
+                        }
+                    });
+                }
             }
         }