Null check NavigationHandler before calling onDown

Prior to the refactor in https://crrev.com/c/2288909 the navigation
handler was null checked. Since the handler is now owned by a
new coordinator, the coordinator is checked instead of the handler
itself, this patch re-adds that extra layer of checking.

Bug: 1113548
Change-Id: I29e938ead6ed11a71e46441b3e00c7ccab13ab23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343575Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796352}

chrome/android/java/src/org/chromium/chrome/browser/gesturenav/HistoryNavigationCoordinator.java

@@ -322,10 +322,8 @@ public class HistoryNavigationCoordinator
      * Starts preparing an edge swipe gesture.
      */
     public void startGesture() {
-        assert mNavigationHandler != null;
-
         // Simulates the initial onDown event to update the internal state.
-        mNavigationHandler.onDown();
+        if (mNavigationHandler != null) mNavigationHandler.onDown();
     }
 
     /**