Work on getting things built with -Wimplicit-fallthrough. This CL was uploaded by git cl split.

Bug: 177475
R=estade@chromium.org

Change-Id: I4f3b6133bb2c4ea46f3db57482c96351bdb3e551
Reviewed-on: https://chromium-review.googlesource.com/889763Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532103}

ui/native_theme/native_theme_aura.cc

@@ -148,7 +148,7 @@ void NativeThemeAura::PaintArrowButton(cc::PaintCanvas* canvas,
       break;
     case kHovered:
       bg_color = SkColorSetRGB(0xD2, 0xD2, 0xD2);
-    // Fall through.
+      FALLTHROUGH;
     case kNormal:
       arrow_color = SkColorSetRGB(0x50, 0x50, 0x50);
       break;