Make OSWindow destructor virtual.

BUG=angle:734

Change-Id: I6deb639abc26a314dd890189613e0a3a2e1be1d2
Reviewed-on: https://chromium-review.googlesource.com/214714
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
diff --git a/util/OSWindow.cpp b/util/OSWindow.cpp
index c649dc2..a637a55 100644
--- a/util/OSWindow.cpp
+++ b/util/OSWindow.cpp
@@ -12,6 +12,9 @@
 {
 }
 
+OSWindow::~OSWindow()
+{}
+
 int OSWindow::getWidth() const
 {
     return mWidth;
diff --git a/util/OSWindow.h b/util/OSWindow.h
index 924ee30..f83514f 100644
--- a/util/OSWindow.h
+++ b/util/OSWindow.h
@@ -17,6 +17,7 @@
 {
   public:
     OSWindow();
+    virtual ~OSWindow();
 
     virtual bool initialize(const std::string &name, size_t width, size_t height) = 0;
     virtual void destroy() = 0;