[remoting linux host] Fix X11 clipboard.

This adds a missing AddEventObserver() call in ClipboardX11.

Bug: 1165520
Change-Id: I20335ab29d4b88b0fff70450074e8fcf951a55d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631327
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Erik Jensen <rkjnsn@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843829}

remoting/host/clipboard_x11.cc

@@ -56,6 +56,9 @@ class ClipboardX11 : public Clipboard, public x11::EventObserver {
 ClipboardX11::ClipboardX11() = default;
 
 ClipboardX11::~ClipboardX11() {
+  if (connection_) {
+    connection_->RemoveEventObserver(this);
+  }
   x_connection_watch_controller_ = nullptr;
 }
 
@@ -68,6 +71,7 @@ void ClipboardX11::Start(
     LOG(ERROR) << "Couldn't open X display";
     return;
   }
+  connection_->AddEventObserver(this);
   client_clipboard_.swap(client_clipboard);
 
   x_server_clipboard_.Init(