arc: Implement proper Arc window activation for task moving to front.

components/exo/shell_surface.cc

Index: components/exo/shell_surface.cc
diff --git a/components/exo/shell_surface.cc b/components/exo/shell_surface.cc
index 0c3a05c0d816439ae5059d805d55426eaa18a98f..8db2308a32b7a707932410169434f87b4936217a 100644
--- a/components/exo/shell_surface.cc
+++ b/components/exo/shell_surface.cc
@@ -335,6 +335,15 @@ void ShellSurface::SetParent(ShellSurface* parent) {
   }
 }
 
+void ShellSurface::Activate() {
+  TRACE_EVENT0("exo", "ShellSurface::Activate");
+
+  if (!widget_ || widget_->IsActive())
+    return;
+
+  widget_->Activate();
+}
+
 void ShellSurface::Maximize() {
   TRACE_EVENT0("exo", "ShellSurface::Maximize");