ui: Add libsync GLFence implementation.

ui/gfx/shared_event.h

Index: ui/gfx/shared_event.h
diff --git a/ui/gfx/shared_event.h b/ui/gfx/shared_event.h
index 1c93a033d41ca4575f5a463eb9ea110f035c2755..d3eda55c4f2a455ee1bc96e42aec9b07afc96f62 100644
--- a/ui/gfx/shared_event.h
+++ b/ui/gfx/shared_event.h
@@ -30,6 +30,10 @@ class GFX_EXPORT SharedEvent {
   // responsible for destroying the OS primitive.
   static SharedEventHandle CreateForProcess(base::ProcessHandle process);
 
+  // Return true iff the given handle is valid (i.e. not the distingished
+  // invalid value; NULL for a HANDLE and -1 for a file descriptor)
+  static bool IsHandleValid(const SharedEventHandle& handle);
+
   // Duplicates the underlying OS primitive. The caller is responsible for
   // destroying the duplicated OS primitive.
   static SharedEventHandle DuplicateHandle(const SharedEventHandle& handle);