Android: Fix NPE in SwipeRefreshHandler

In order to prevent a runnable |mDetachRefreshLayoutRunnable| from
being invoked after |SwipeRefreshLayout| is set to zero, this CL adds
a missing null check.

Bug: 1045952
Change-Id: I48bf09831a42dccda3f38b1101184a27caf0218c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028870
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736767}

chrome/android/java/src/org/chromium/chrome/browser/SwipeRefreshHandler.java

@@ -152,7 +152,7 @@ public class SwipeRefreshHandler
     @SuppressLint("NewApi")
     @Override
     public void cleanupWebContents(WebContents webContents) {
-        if (mSwipeRefreshLayout != null) detachSwipeRefreshLayoutIfNecessary();
+        detachSwipeRefreshLayoutIfNecessary();
         mContainerView = null;
         mNavigationHandler = null;
         setEnabled(false);
@@ -274,6 +274,7 @@ public class SwipeRefreshHandler
     }
 
     private void detachSwipeRefreshLayoutIfNecessary() {
+        if (mSwipeRefreshLayout == null) return;
         cancelDetachLayoutRunnable();
         if (mSwipeRefreshLayout.getParent() != null) {
             mContainerView.removeView(mSwipeRefreshLayout);