Add libgtk3ui to gn_all (Reland)

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 0fa9b598754c23c08eefcb039aa529a6752b8197..1e56a6c15c33941d11519141ff1941ea7e680574 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -357,6 +357,12 @@ group("both_gn_and_gyp") {
     ]
   }
 
+  if (is_linux && !is_chromeos && !is_chromecast && !use_ozone) {
+    # TODO(thomasanderson): Remove this once we build using
+    # GTK3 by default. (crbug.com/132847, crbug.com/79722)
+    deps += [ "//chrome/browser/ui/libgtkui:libgtk3ui" ]
+  }
+
   if (use_ozone) {
     deps += [ "//ui/ozone" ]
   }