[android_webview] Fix crash in testCreateDestroy.

Cherry-pick of http://crrev.com/28923005

> The onSizeChanged callback was arriving after the AwContents was destroyed.
> Ignoring the setFixedLayoutSize call is fine since the AwContents is going
> away anyway.
>
> BUG=308497
> R=mnaganov@chromium.org
>
> Review URL: https://codereview.chromium.org/28923005
>
> git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229798 0039d316-1c4b-4281-b951-d872f2087c98

BUG: 11316652
Change-Id: I6e29e143aa5b14389df3d55756fc8bda315f245d
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 09c337f..be362a3 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -317,6 +317,7 @@
 
         @Override
         public void setFixedLayoutSize(int widthDip, int heightDip) {
+            if (mNativeAwContents == 0) return;
             nativeSetFixedLayoutSize(mNativeAwContents, widthDip, heightDip);
         }
     }