Null-check navigationController before using

Context:
WebContents#getNavigationController() can return null.
PaymentHandlerMediator should null-check the getter's return before
using it.

Bug: 1137695

Change-Id: I9a78ebfd60666268ef48566e0d7437a59e198a3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463713Reviewed-by: Nick Burris <nburris@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820034}

chrome/android/java/src/org/chromium/chrome/browser/payments/handler/PaymentHandlerMediator.java

@@ -286,6 +286,6 @@ import java.lang.annotation.RetentionPolicy;
 
     private void onSystemBackButtonClicked() {
         NavigationController navigation = mPaymentHandlerWebContents.getNavigationController();
-        if (navigation.canGoBack()) navigation.goBack();
+        if (navigation != null && navigation.canGoBack()) navigation.goBack();
     }
 }