mus: Use the new drag API to implement tab dragging in chrome.

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index 96fc131e75fe32f7df574626a644fb688dbebe1f..15b41e45c7fddbfd8c93d096c1579cf5a2d644cb 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -330,6 +330,13 @@ target(chrome_browser_target_type, "ui") {
           "//components/mus/public/cpp",
           "//components/mus/public/interfaces",
         ]
+
+        # TODO(erg): These files hard depend on mus, and thus can't be in a gyp
+        # build. When gyp goes away, merge this back into the sources list.
+        sources += [
+          "views/tabs/window_finder_mus.cc",
+          "views/tabs/window_finder_mus.h",
+        ]
       }
       if (enable_extensions) {
         sources += rebase_path(