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 d8e6346e254c7587e3d01f24df780963ee398b11..4776066c58f25d7898a06ab7d249e76dc768e4e5 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -324,6 +324,13 @@ split_static_library("ui") {
           "//services/ui/public/cpp",
           "//services/ui/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(