WebGL: Don't destroy mailbox textures in the destructor until they're released.

Source/web/tests/DrawingBufferTest.cpp

Index: Source/web/tests/DrawingBufferTest.cpp
diff --git a/Source/web/tests/DrawingBufferTest.cpp b/Source/web/tests/DrawingBufferTest.cpp
index 9cd71920c97e6d98bcaeb2a36c42a1570a6c8af1..1ecffbd4dfcc672c1ad764d74364414e2580ecdf 100644
--- a/Source/web/tests/DrawingBufferTest.cpp
+++ b/Source/web/tests/DrawingBufferTest.cpp
@@ -112,16 +112,17 @@ protected:
     virtual void SetUp()
     {
         RefPtr<FakeContextEvictionManager> contextEvictionManager = adoptRef(new FakeContextEvictionManager());
-        m_context = adoptPtr(new WebGraphicsContext3DForTests);
-        m_drawingBuffer = DrawingBuffer::create(m_context.get(), IntSize(initialWidth, initialHeight), DrawingBuffer::Preserve, contextEvictionManager.release());
+        OwnPtr<WebGraphicsContext3DForTests> context = adoptPtr(new WebGraphicsContext3DForTests);
+        m_context = context.get();
+        m_drawingBuffer = DrawingBuffer::create(context.release(), IntSize(initialWidth, initialHeight), DrawingBuffer::Preserve, contextEvictionManager.release());
     }
 
     WebGraphicsContext3DForTests* webContext()
     {
-        return m_context.get();
+        return m_context;
     }
 
-    OwnPtr<WebGraphicsContext3DForTests> m_context;
+    WebGraphicsContext3DForTests* m_context;
     RefPtr<DrawingBuffer> m_drawingBuffer;
 };