Revise tab spacing in vertical mode

This cl revises the tab spacing in vertical mode to match the spacing in
the wallpaper picker.

Images before and after changes are uploaded in the bug.

Bug: 889559
Test: manual.
Change-Id: I33a73809a9c674b009c5fdf1f9367645d395af87
Reviewed-on: https://chromium-review.googlesource.com/1249818Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594896}

ui/views/controls/tabbed_pane/tabbed_pane.cc

@@ -407,7 +407,7 @@ TabStrip::TabStrip(TabbedPane::Orientation orientation,
     layout->set_cross_axis_alignment(BoxLayout::CROSS_AXIS_ALIGNMENT_END);
   } else {
     const int kTabStripEdgePadding = 8;
-    const int kTabSpacing = 16;
+    const int kTabSpacing = 8;
     layout = std::make_unique<BoxLayout>(
         BoxLayout::kVertical, gfx::Insets(kTabStripEdgePadding, 0, 0, 0),
         kTabSpacing);