Clear the clipboard to claim ownership of it before updating it.

Bug: 879729
Change-Id: I8a7296b344c42115a7fade7d965a6b38f0688f59
Reviewed-on: https://chromium-review.googlesource.com/c/1281239Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599742}

remoting/host/clipboard_mac.mm

@@ -80,6 +80,7 @@ void ClipboardMac::InjectClipboardEvent(const protocol::ClipboardEvent& event) {
   // Write text to clipboard.
   NSString* text = base::SysUTF8ToNSString(event.data());
   NSPasteboard* pasteboard = [NSPasteboard generalPasteboard];
+  [pasteboard clearContents];
   [pasteboard writeObjects:@[ text ]];
 
   // Update local change-count to prevent this change from being picked up by