[ozone] Fixed compilation for Ozone and X11.

The recently landed change [1] has introduced a pure virtual method for
ui::Clipboard on Ozone, which has broken the hybrid build that has
USE_OZONE and USE_X11 set together.

This CL fixes that by adding a stub implementation of that method to
ui::ClipboardX11 with Ozone enabled.

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

Change-Id: Ia93c8b488f7ffd2009200f894acd6d77ccdb1351
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346671Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#798706}

ui/base/clipboard/clipboard_x11.cc

@@ -709,6 +709,12 @@ void ClipboardX11::ReadData(const ClipboardFormatType& format,
     data.AssignTo(result);
 }
 
+#if defined(USE_OZONE)
+bool ClipboardX11::IsSelectionBufferAvailable() const {
+  return true;
+}
+#endif  // defined(USE_OZONE)
+
 // |data_src| is not used. It's only passed to be consistent with other
 // platforms.
 void ClipboardX11::WritePortableRepresentations(

ui/base/clipboard/clipboard_x11.h

@@ -65,6 +65,9 @@ class ClipboardX11 : public Clipboard {
   void ReadData(const ClipboardFormatType& format,
                 const ClipboardDataEndpoint* data_dst,
                 std::string* result) const override;
+#if defined(USE_OZONE)
+  bool IsSelectionBufferAvailable() const override;
+#endif  // defined(USE_OZONE)
   void WritePortableRepresentations(
       ClipboardBuffer buffer,
       const ObjectMap& objects,