Cherry-pick 269805@main (d64c4de). rdar://119608829

    [cairo] Crash in PathStream::copy() after 268923@main
    https://bugs.webkit.org/show_bug.cgi?id=263666

    Reviewed by Chris Dumez.

    This happens sometimes when threaded rendering in enabled. We need to
    make PathImpl reference count thread safe. Fix spotted by Chris Dumez.

    * Source/WebCore/platform/graphics/PathImpl.h:

    Canonical link: https://commits.webkit.org/269805@main

Canonical link: https://commits.webkit.org/267815.631@safari-7617-branch

Source/WebCore/platform/graphics/PathImpl.h

@@ -29,12 +29,12 @@
 #include "PathElement.h"
 #include "PathSegment.h"
 #include <wtf/FastMalloc.h>
-#include <wtf/RefCounted.h>
+#include <wtf/ThreadSafeRefCounted.h>
 #include <wtf/UniqueRef.h>
 
 namespace WebCore {
 
-class PathImpl : public RefCounted<PathImpl> {
+class PathImpl : public ThreadSafeRefCounted<PathImpl> {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     virtual ~PathImpl() = default;