WebGL: Fix assertion hit of gpu tests.

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index e0254864410b0dfe8179a2fd76eea549e89f1293..4967200760aa47b3e78f2992546dea0e3f31bb55 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -191,12 +191,18 @@ blink::WebGraphicsContext3D* DrawingBuffer::context()
 
 bool DrawingBuffer::prepareMailbox(blink::WebExternalTextureMailbox* outMailbox, blink::WebExternalBitmap* bitmap)
 {
-    // prepareMailbox() is always called after layout.
-    ASSERT(!m_destructionInProgress);
-
     if (!m_contentsChanged)
         return false;
 
+    if (m_destructionInProgress) {
+        // It can be hit in the following sequence.
+        // 1. WebGL draws something.
+        // 2. The compositor begins the frame.
+        // 3. Javascript makes a context lost using WEBGL_lose_context extension.
+        // 4. Here.
+        return false;
+    }
+
     m_context->makeContextCurrent();
 
     // Resolve the multisampled buffer into m_colorBuffer texture.