Zero-check mNativePointer in AppBannerUiDelegateAndroid.

This is a speculative fix for crashes caused by native methods being
called after this object is destroyed.

BUG=871129

Change-Id: Id620402711979e6e380dd8e307cca098619e962d
Reviewed-on: https://chromium-review.googlesource.com/1163407Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581020}

chrome/android/java/src/org/chromium/chrome/browser/banners/AppBannerUiDelegateAndroid.java

@@ -42,17 +42,21 @@ public class AppBannerUiDelegateAndroid
     public void addToHomescreen(String title) {
         mAddedToHomescreen = true;
         // The title is ignored for app banners as we respect the developer-provided title.
-        nativeAddToHomescreen(mNativePointer);
+        if (mNativePointer != 0) {
+            nativeAddToHomescreen(mNativePointer);
+        }
     }
 
     @Override
     public void onNativeAppDetailsRequested() {
-        nativeShowNativeAppDetails(mNativePointer);
+        if (mNativePointer != 0) {
+            nativeShowNativeAppDetails(mNativePointer);
+        }
     }
 
     @Override
     public void onDialogDismissed() {
-        if (!mAddedToHomescreen) {
+        if (!mAddedToHomescreen && mNativePointer != 0) {
             nativeOnUiCancelled(mNativePointer);
         }