[WebKit2] CustomProtocolManager's m_registeredSchemes HashSet can be accessed by multiple threads
https://bugs.webkit.org/show_bug.cgi?id=115267

Reviewed by Alexey Proskuryakov.

* Shared/Network/CustomProtocols/CustomProtocolManager.h: Added a Mutex to protect m_registeredSchemes.
* Shared/Network/CustomProtocols/mac/CustomProtocolManagerMac.mm:
(WebKit::CustomProtocolManager::registerScheme): Acquired the mutex before accessing m_registeredSchemes.
(WebKit::CustomProtocolManager::unregisterScheme): Ditto.
(WebKit::CustomProtocolManager::supportsScheme): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149198 268f45cc-cd09-0410-ab3c-d52691b4dbfc