[Autofill Assistant] Move static member to cc.

Before this patch, building desktop chrome using code from
autofill_assistant doesn't link. This patch moves the static member in
wait for navigation to the cc file to resolve that.

R=sandromaggi@google.com

Bug: b/143736397
Change-Id: I9c0c70657037b66316dff42d07494c6536956596
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975530Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726320}

components/autofill_assistant/browser/actions/wait_for_navigation_action.cc

@@ -10,6 +10,9 @@
 #include "components/autofill_assistant/browser/actions/action_delegate.h"
 
 namespace autofill_assistant {
+namespace {
+constexpr base::TimeDelta kDefaultTimeout = base::TimeDelta::FromSeconds(20);
+}  // namespace
 
 WaitForNavigationAction::WaitForNavigationAction(ActionDelegate* delegate,
                                                  const ActionProto& proto)

components/autofill_assistant/browser/actions/wait_for_navigation_action.h

@@ -22,9 +22,6 @@ class WaitForNavigationAction : public Action {
   ~WaitForNavigationAction() override;
 
  private:
-  static constexpr base::TimeDelta kDefaultTimeout =
-      base::TimeDelta::FromSeconds(20);
-
   // Overrides Action:
   void InternalProcessAction(ProcessActionCallback callback) override;