Instantiate WebContentsAX only when WebContents is ready

The reported crash indicates that WebContents object is null
when WebContentsAccessibility object gets instantiated.
This may happen if AccessibilityNodeProvider is requested
after WebContents/CVC get destroyed. In which case we should
not proceed to creating one but return null instead.

BUG=749761

Change-Id: If74945442fd6d05f2f2090c6e0a6aca375fb04f2
Reviewed-on: https://chromium-review.googlesource.com/590887Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490698}

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

@@ -1930,6 +1930,7 @@ public class ContentViewCore implements AccessibilityStateChangeListener, Displa
             }
             mWebContentsAccessibility.enable();
         } else if (mNativeAccessibilityAllowed) {
+            if (mWebContents == null) return null;
             mWebContentsAccessibility = WebContentsAccessibility.create(mContext, mContainerView,
                     mWebContents, mRenderCoordinates, mShouldSetAccessibilityFocusOnPageLoad);
             mWebContentsAccessibility.enable();