Add Negative Padding for active slider to merge slider and thumb

Test: Manually Tested UI

Change-Id: Ia64891de1bceb262e4ad7f9fe5732fb4bcf34109
Bug: 951488
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1561848
Auto-Submit: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649691}

ui/views/controls/slider.cc

@@ -270,9 +270,10 @@ void Slider::OnPaint(gfx::Canvas* canvas) {
       is_active_ ? kEmptySliderColor
                  : SkColorSetA(kEmptySliderColor, kHighlightColorAlpha);
 
-  // Extra space used to hide slider ends behind the thumb when slider is
-  // disabled.
-  const int extra_padding = is_active_ ? 0 : kSliderPadding;
+  // Padding used to adjust space between slider ends and slider thumb.
+  // Value is negative when slider is active so that there is no separation
+  // between slider and thumb.
+  const int extra_padding = is_active_ ? -kSliderPadding : kSliderPadding;
 
   cc::PaintFlags slider_flags;
   slider_flags.setAntiAlias(true);