[aw] Fix crash when VisualStateCallback is null

This CL fixes a crash when a null VisualStateCallback is passed to
postVisualStateCallback. Since there is no action to do if the callback
is null, just return early.

Bug: 1014924
Change-Id: I891eda56bd884af5b2d7e1cb909895cb5fdd6a98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865155Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Laís Minchillo <laisminchillo@chromium.org>
Auto-Submit: Laís Minchillo <laisminchillo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706438}

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromium.java

@@ -958,6 +958,7 @@ class WebViewChromium implements WebViewProvider, WebViewProvider.ScrollDelegate
     public void insertVisualStateCallback(
             final long requestId, final VisualStateCallback callback) {
         sWebViewApiCallSample.record(ApiCall.INSERT_VISUAL_STATE_CALLBACK);
+        if (callback == null) return;
         mSharedWebViewChromium.insertVisualStateCallback(
                 requestId, new AwContents.VisualStateCallback() {
                     @Override