Cherry-pick: [Android WebView] Null check syncOnNewPictureStateToNative

Cherry-pick of Chromium crrev.com/r233453

BUG: 11555111

Original description:

Make sure we don't call into native after the native pointer has been
destroyed.

Change-Id: Id0321a74656fd5a83d3e82115638e65c08531a10
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContents.java b/android_webview/java/src/org/chromium/android_webview/AwContents.java
index c880873..3278ef5 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -816,6 +816,7 @@
     }
 
     private void syncOnNewPictureStateToNative() {
+        if (mNativeAwContents == 0) return;
         nativeEnableOnNewPicture(mNativeAwContents, mPictureListenerEnabled || mClearViewActive);
     }