Ignore brightness change less than threshold.

Previously, brightness change by slider less than the threshold was not
ignored in UnifiedSystemTray, which led to some artifacts (see the
video in the bug.)

TEST=manual
BUG=858948

Change-Id: I14dcbe6f02e0b35cb88b468421ca8416b9285dbe
Reviewed-on: https://chromium-review.googlesource.com/1124210Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572474}

ash/system/brightness/unified_brightness_slider_controller.cc

@@ -37,13 +37,17 @@ void UnifiedBrightnessSliderController::SliderValueChanged(
     views::SliderChangeReason reason) {
   if (reason != views::VALUE_CHANGED_BY_USER)
     return;
+
   BrightnessControlDelegate* brightness_control_delegate =
       Shell::Get()->brightness_control_delegate();
-  if (brightness_control_delegate) {
-    double percent =
-        std::max<float>(value * 100.f, tray::kMinBrightnessPercent);
-    brightness_control_delegate->SetBrightnessPercent(percent, true);
-  }
+  if (!brightness_control_delegate)
+    return;
+
+  double percent = value * 100.;
+  if (percent < tray::kMinBrightnessPercent)
+    return;
+
+  brightness_control_delegate->SetBrightnessPercent(percent, true);
 }
 
 }  // namespace ash