Fix scrollbar buttons on ChromeOS.

ui/native_theme/native_theme_base.cc

Index: ui/native_theme/native_theme_base.cc
diff --git a/ui/native_theme/native_theme_base.cc b/ui/native_theme/native_theme_base.cc
index 548b4a5cf695869c665d8eee425948b31e0d60af..ad80228c5d49aa4ca6894bdba57cb98fe78a69ae 100644
--- a/ui/native_theme/native_theme_base.cc
+++ b/ui/native_theme/native_theme_base.cc
@@ -115,10 +115,9 @@ gfx::Size NativeThemeBase::GetPartSize(Part part,
       return gfx::Size(kCheckboxAndRadioWidth, kCheckboxAndRadioHeight);
     case kScrollbarDownArrow:
     case kScrollbarUpArrow:
-      return gfx::Size(scrollbar_width_, scrollbar_button_length_);
     case kScrollbarLeftArrow:
     case kScrollbarRightArrow:
-      return gfx::Size(scrollbar_button_length_, scrollbar_width_);

[Evan Stade, 2016/05/23 22:30:59]

why not leave most stuff alone, but change scrollbar_width_ to 0 if length is 0?

[Bret, 2016/05/23 22:53:15]

That wasn't actually the problem. I thought this was unused which is why I
removed it, but you're right it looks like native_scroll_bar_views.cc is using
it. I'll change this back.

[Bret, 2016/05/23 23:44:06]

Done.

+      return gfx::Size();  // No default size.
     case kScrollbarHorizontalThumb:
       // This matches Firefox on Linux.
       return gfx::Size(2 * scrollbar_width_, scrollbar_width_);
@@ -221,7 +220,7 @@ void NativeThemeBase::Paint(SkCanvas* canvas,
     case kScrollbarUpArrow:
     case kScrollbarLeftArrow:
     case kScrollbarRightArrow:
-      if (scrollbar_button_length_ > 0)
+      if (HasScrollbarButtons())

[esprehn, 2016/05/23 23:11:19]

how did we get here if it was zero size?

[Bret, 2016/05/23 23:44:06]

It looks like ScrollbarThemeAura just calls paint for every piece of the
scrollbar without checking their sizes.

         PaintArrowButton(canvas, rect, part, state);
       break;
     case kScrollbarHorizontalThumb:
@@ -265,10 +264,7 @@ void NativeThemeBase::Paint(SkCanvas* canvas,
   canvas->restore();
 }
 
-NativeThemeBase::NativeThemeBase()
-    : scrollbar_width_(kDefaultScrollbarWidth),
-      scrollbar_button_length_(kDefaultScrollbarButtonLength) {
-}
+NativeThemeBase::NativeThemeBase() : scrollbar_width_(kDefaultScrollbarWidth) {}
 
 NativeThemeBase::~NativeThemeBase() {
 }