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

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index d9a684713346b45c30412904a0e509eb76b010da..123ae27dba8f9e6b4a49bf008d0b76dffdf5fde8 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -721,6 +721,7 @@ void WebGLRenderingContextBase::destroyContext()
     webContext()->setErrorMessageCallback(0);
 
     ASSERT(m_drawingBuffer);
+    m_drawingBuffer->beginDestruction();
     m_drawingBuffer.clear();
 }
 
@@ -5372,7 +5373,10 @@ void WebGLRenderingContextBase::maybeRestoreContext(Timer<WebGLRenderingContextB
         return;
 
     // If the context was lost due to RealLostContext, we need to destroy the old DrawingBuffer before creating new DrawingBuffer to ensure resource budget enough.
-    m_drawingBuffer.clear();
+    if (m_drawingBuffer) {
+        m_drawingBuffer->beginDestruction();
+        m_drawingBuffer.clear();
+    }
 
     blink::WebGraphicsContext3D::Attributes attributes = m_requestedAttributes->attributes(canvas()->document().topDocument().url().string(), settings);
     OwnPtr<blink::WebGraphicsContext3D> context = adoptPtr(blink::Platform::current()->createOffscreenGraphicsContext3D(attributes, 0));