Reduce Slider thumb size to 8dip diameter

Test: Manually tested UI
Change-Id: Ic92814101b9b8fd25c77ef9fb2f44b1531f7ceac
Bug: 951486
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562223
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649694}

ui/views/controls/slider.cc

@@ -51,9 +51,9 @@ constexpr int kSliderPadding = 2;
 
 // The radius of the thumb and the highlighted thumb of the slider,
 // respectively.
-constexpr float kThumbRadius = 6.f;
+constexpr float kThumbRadius = 4.f;
 constexpr float kThumbWidth = 2 * kThumbRadius;
-constexpr float kThumbHighlightRadius = 10.f;
+constexpr float kThumbHighlightRadius = 12.f;
 
 // Duration of the thumb highlight growing effect animation.
 constexpr int kSlideHighlightChangeDurationMs = 150;