Reset the clipboard writer before destroying the clipboard message filter.

We cannot commit them at this point because we are running on the IO thread.

BUG=465586

Review URL: https://codereview.chromium.org/987583005

Cr-Commit-Position: refs/heads/master@{#319838}

content/browser/renderer_host/clipboard_message_filter.cc

@@ -91,6 +91,7 @@ bool ClipboardMessageFilter::OnMessageReceived(const IPC::Message& message) {
 }
 
 ClipboardMessageFilter::~ClipboardMessageFilter() {
+  clipboard_writer_->Reset();
 }
 
 void ClipboardMessageFilter::OnGetSequenceNumber(ui::ClipboardType type,