Rewrite clipboard write IPC handling to be easier to understand.

ui/base/test/test_clipboard.h

Index: ui/base/test/test_clipboard.h
diff --git a/ui/base/test/test_clipboard.h b/ui/base/test/test_clipboard.h
index c1701bf72464242fe041bd3aa538d3b7a161fd67..58fa5075fa613abe76ea593552769cdf26013da6 100644
--- a/ui/base/test/test_clipboard.h
+++ b/ui/base/test/test_clipboard.h
@@ -16,7 +16,10 @@ class TestClipboard : public Clipboard {
   TestClipboard();
   ~TestClipboard() override;
 
-  static void UseForCurrentThread();
+  // Creates and associates a TestClipboard with the current thread. When no
+  // longer needed, the returned clipboard must be freed by calling
+  // Clipboard::DestroyClipboardForCurrentThread() on the same thread.
+  static Clipboard* CreateForCurrentThread();
 
   // Clipboard overrides.
   uint64 GetSequenceNumber(ClipboardType type) const override;