Add missing includes for threads.h

The missing headers were being transitively included through a
Skia header which is being removed.

Change-Id: I6a5b26c1f1bb3ed4c65e7e39fff3b927cb14eabf
diff --git a/samples/BrowserPlugin/jni/RenderingThread.h b/samples/BrowserPlugin/jni/RenderingThread.h
index cb4fb96..6136019 100644
--- a/samples/BrowserPlugin/jni/RenderingThread.h
+++ b/samples/BrowserPlugin/jni/RenderingThread.h
@@ -26,6 +26,7 @@
 #include "SkCanvas.h"
 #include "SkBitmap.h"
 
+#include <utils/threads.h>
 #include <EGL/egl.h>
 #include <GLES2/gl2.h>