Android: Guard against crashes due to accessing invalid objects

The reported crashes likely come from accessing objects in
WebContents when it's on the way to destruction. Adds a check to
ensure it won't attempt to access objects that already become invalid.

Bug: 788278
Change-Id: I8d4ef4907c73a93ac4d6d12e78540bbbbfbae08e
Reviewed-on: https://chromium-review.googlesource.com/1027170
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553808}

content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsImpl.java

@@ -685,6 +685,7 @@ public class WebContentsImpl implements WebContents, RenderFrameHostDelegate {
      * Removes handles used in text selection.
      */
     public void dismissTextHandles() {
+        if (isDestroyed()) return;
         nativeDismissTextHandles(mNativeWebContentsAndroid);
     }