Don't draw toolbar strokes for web app windows that don't have a tab strip

These windows should let the title bar bleed into the WebContents below
without any separator.
Web app windows with a title bar require a separator line to show the
outline of any tabs when adjacent colors are too similar.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=475568&signed_aid=70iR6Zl6eyaEks4wOpBBZg==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=475569&signed_aid=MoU4hgqV5NSMUiizHK5tsw==&inline=1

This CL effectively reverts the change made in
https://chromium-review.googlesource.com/c/chromium/src/+/2367584/5/chrome/browser/ui/views/frame/browser_non_client_frame_view.cc
to only allow strokes for tab strip web apps instead of all web apps.

Bug: 1137301
Change-Id: Ie0dd29872765921f7c1c8840c39bd70736423bed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526756Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825731}

chrome/browser/ui/views/frame/browser_non_client_frame_view.cc

@@ -141,7 +141,9 @@ bool BrowserNonClientFrameView::EverHasVisibleBackgroundTabShapes() const {
 }
 
 bool BrowserNonClientFrameView::CanDrawStrokes() const {
-  return true;
+  // Web apps should not draw strokes if they don't have a tab strip.
+  return !browser_view_->browser()->app_controller() ||
+         browser_view_->browser()->app_controller()->has_tab_strip();
 }
 
 SkColor BrowserNonClientFrameView::GetCaptionColor(