[ozone/x11] Fixed the MIME type matching.

A recent improvement of memory usage in ozone/x11 clipboard [1]
introduced a regression: copying the text stopped working because the
standard MIME type for the UTF-8 encoding was not inserted into the
map.  This CL fixes that.

[1] https://crrev.com.com/c/2292396

Change-Id: I5cedb36253a4f2be392e937f62781417eead8ad3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306252
Auto-Submit: Alexander Dunaev <adunaev@igalia.com>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789947}

ui/base/clipboard/clipboard_ozone.cc

@@ -556,7 +556,7 @@ void ClipboardOzone::WriteText(const char* text_data, size_t text_len) {
   std::vector<uint8_t> data(text_data, text_data + text_len);
   async_clipboard_ozone_->InsertData(
       std::move(data), {kMimeTypeText, kMimeTypeX11Text, kMimeTypeX11String,
-                        kMimeTypeX11Utf8String});
+                        kMimeTypeTextUtf8, kMimeTypeX11Utf8String});
 }
 
 void ClipboardOzone::WriteHTML(const char* markup_data,