Initialize |switch_profile_link_| in the constructor.

This should fix the use of the uninitialized value in the LinkClicked() method.
Currently, the variable is only initialized for supervised user profiles,
but LinkClicked() is also called for other profiles.

BUG=422482

Review URL: https://codereview.chromium.org/639163003

Cr-Commit-Position: refs/heads/master@{#299672}

Author: akuegel

chrome/browser/ui/views/profiles/avatar_menu_bubble_view.cc

@@ -517,6 +517,7 @@ AvatarMenuBubbleView::AvatarMenuBubbleView(
       buttons_view_(NULL),
       supervised_user_info_(NULL),
       separator_switch_users_(NULL),
+      switch_profile_link_(nullptr),
       expanded_(false) {
   avatar_menu_.reset(new AvatarMenu(
       &g_browser_process->profile_manager()->GetProfileInfoCache(),