ImageBuffers can be RemoteImageBufferProxy instances that are only safe to use on the thread they were created on. We want to be able to transfer the reference to a new thread, so this adds SerializedImageBuffer to represent the state where we're currently transferring between threads.
Pull request: https://github.com/WebKit/WebKit/pull/7511
<rdar://problem/103531451>
This got subsumed into bug 244830 *** This bug has been marked as a duplicate of bug 244830 ***