Base the tab separator color on the tab background color.

Bug: 862664
Change-Id: I05e936b9f165278d46f7b9aee4dc1fe8dd20624c
Reviewed-on: https://chromium-review.googlesource.com/1139410
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578937}

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

@@ -118,11 +118,15 @@ SkColor BrowserNonClientFrameView::GetToolbarTopSeparatorColor() const {
 
 SkColor BrowserNonClientFrameView::GetTabSeparatorColor() const {
   DCHECK(MD::IsRefreshUi());
+  // TODO(pkasting): https://crbug.com/866689  Replace this with "blend
+  // background tab text color toward background tab background color at 46% or
+  // 1.84 contrast ratio, whichever is larger" (and make sure the blend is large
+  // enough in the default theme to not need adjusting).
   constexpr SkAlpha kTabSeparatorAlpha = 0x4D;  // 30%
-  const SkColor frame_color = GetFrameColor();
+  const SkColor tab_color = GetTabBackgroundColor(TAB_INACTIVE);
   const SkColor base_color =
-      color_utils::BlendTowardOppositeLuma(frame_color, SK_AlphaOPAQUE);
-  return color_utils::AlphaBlend(base_color, frame_color, kTabSeparatorAlpha);
+      color_utils::BlendTowardOppositeLuma(tab_color, SK_AlphaOPAQUE);
+  return color_utils::AlphaBlend(base_color, tab_color, kTabSeparatorAlpha);
 }
 
 SkColor BrowserNonClientFrameView::GetTabBackgroundColor(TabState state) const {