Fix scrollbar buttons at hidpi when enable-use-zoom-for-dsf is on.

third_party/WebKit/Source/platform/BUILD.gn

Index: third_party/WebKit/Source/platform/BUILD.gn
diff --git a/third_party/WebKit/Source/platform/BUILD.gn b/third_party/WebKit/Source/platform/BUILD.gn
index 2527b49695bb7e984a0aed67cf8f572025805684..f1a55ff988abde04262adae281491c218d3d00ce 100644
--- a/third_party/WebKit/Source/platform/BUILD.gn
+++ b/third_party/WebKit/Source/platform/BUILD.gn
@@ -544,6 +544,10 @@ test("blink_platform_unittests") {
     sources += [ "scroll/ScrollAnimatorTest.cpp" ]
   }
 
+  if (use_default_render_theme) {
+    sources += [ "scroll/ScrollbarThemeAuraTest.cpp" ]
+  }
+
   sources += [ "testing/RunAllTests.cpp" ]
 
   configs += [