Use themed vector icon as avatar placeholder

Replaces the png graphics that blends into the default-color highlight
effect.

Bug: chromium:978388

Change-Id: I33e63b1e15c225761f6f089aac5e853463169b9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1736440Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684401}

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

@@ -326,6 +326,12 @@ bool AvatarToolbarButton::ShouldShowGenericIcon() const {
     // This can happen if the user deletes the current profile.
     return true;
   }
+
+  // If the profile is using the placeholder avatar, fall back on the themeable
+  // vector icon instead.
+  if (entry->GetAvatarIconIndex() == profiles::GetPlaceholderAvatarIndex())
+    return true;
+
   return entry->GetAvatarIconIndex() == 0 &&
          g_browser_process->profile_manager()
                  ->GetProfileAttributesStorage()