Added the missing IPC trait macro.

This is a trivial follow-up to the patch that had been landed in August
[1].  It turned out that the new parameter that had been introduced with
that patch had not been registered properly in the IPC parameter traits,
so its value is not transported properly.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2307234

Change-Id: Idd45ac735c4dd952d4e4a2587175ee6dd5246a4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438498
Auto-Submit: Alexander Dunaev <adunaev@igalia.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812633}

content/public/common/common_param_traits_macros.h

@@ -140,6 +140,7 @@ IPC_STRUCT_TRAITS_BEGIN(blink::mojom::RendererPreferences)
   IPC_STRUCT_TRAITS_MEMBER(caret_browsing_enabled)
 #if defined(OS_LINUX) || defined(OS_CHROMEOS)
   IPC_STRUCT_TRAITS_MEMBER(system_font_family_name)
+  IPC_STRUCT_TRAITS_MEMBER(selection_clipboard_buffer_available)
 #endif
 #if defined(OS_WIN)
   IPC_STRUCT_TRAITS_MEMBER(caption_font_family_name)