android: fix ContentReadbackHandler destroy()

The native component is not guarranteed to be initialized before destroy()
is called.

BUG=375726

Review URL: https://codereview.chromium.org/295033003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272265 0039d316-1c4b-4281-b951-d872f2087c98

content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java

@@ -52,7 +52,7 @@ public abstract class ContentReadbackHandler {
      * Should be called when the ContentReadackHandler is not needed anymore.
      */
     public void destroy() {
-        nativeDestroy(mNativeContentReadbackHandler);
+        if (mNativeContentReadbackHandler != 0) nativeDestroy(mNativeContentReadbackHandler);
         mNativeContentReadbackHandler = 0;
     }