[Avatar button] Make the guest mode avatar consistent with the menu

This CL makes the guest mode icon lighter gray and most notably makes
the color consistent with the user menu by using the same helper
function to create the icon.

Bug: 967317
Change-Id: I0df790897b00533a20558caf9c834db3d15ce363
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829703
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701081}

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

@@ -435,8 +435,7 @@ gfx::ImageSkia AvatarToolbarButton::GetAvatarIcon(
       return gfx::CreateVectorIcon(kIncognitoIcon, icon_size, icon_color);
     case State::kGuestSession:
       if (base::FeatureList::IsEnabled(features::kAnimatedAvatarButton)) {
-        return gfx::CreateVectorIcon(kUserAccountAvatarIcon, icon_size,
-                                     icon_color);
+        return profiles::GetGuestAvatar(icon_size);
       }
       return gfx::CreateVectorIcon(kUserMenuGuestIcon, icon_size, icon_color);
     case State::kGenericProfile: