Fix crash in WebContentsAccessibility

WebContentsAccessibility as the implementation of AccessibilityNodeProvider
can get |createAccessibilityNodeInfo| even past the WebContents finished
its lifecycle. In such case mWebContents should be null-checked to prevent
crash. This CL puts the missing check that caused the reported crash.

Bug: 806033
Change-Id: Ic80a6fd7d6289aab332a60221cd3c7a1c93d12c8
Reviewed-on: https://chromium-review.googlesource.com/887698Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532300}

content/public/android/java/src/org/chromium/content/browser/accessibility/WebContentsAccessibility.java

@@ -736,6 +736,10 @@ public class WebContentsAccessibility extends AccessibilityNodeProvider
      * gets initialized first.
      */
     private boolean isFrameInfoInitialized() {
+        if (mWebContents == null) {
+            // We already got frame info since WebContents finished its lifecycle.
+            return true;
+        }
         RenderCoordinates rc = mWebContents.getRenderCoordinates();
         return rc.getContentWidthCss() != 0.0 || rc.getContentHeightCss() != 0.0;
     }