WebGL: Wait for the sync point that the compositor inserted before deleting a mailbox.

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 4967200760aa47b3e78f2992546dea0e3f31bb55..8927cd7eb2ea07757d87929eff452128fe81650e 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -344,6 +344,8 @@ void DrawingBuffer::deleteMailbox(const blink::WebExternalTextureMailbox& mailbo
 {
     for (size_t i = 0; i < m_textureMailboxes.size(); i++) {
         if (nameEquals(m_textureMailboxes[i]->mailbox, mailbox)) {
+            if (mailbox.syncPoint)
+                m_context->waitSyncPoint(mailbox.syncPoint);
             m_context->deleteTexture(m_textureMailboxes[i]->textureId);
             m_textureMailboxes.remove(i);
             return;