Hide non-composited native scrollbars.

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
index aac9bb4255313c6870002f840f5ac9720a846230..9b7e2930a8401e157619fd97ac55e66e14ed3b59 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
@@ -296,6 +296,10 @@ void ScrollbarTheme::splitTrack(const ScrollbarThemeClient& scrollbar, const Int
 ScrollbarTheme& ScrollbarTheme::theme()
 {
     if (ScrollbarTheme::mockScrollbarsEnabled()) {
+        if (RuntimeEnabledFeatures::hideScrollbarsEnabled()) {
+            DEFINE_STATIC_LOCAL(ScrollbarThemeOverlayMock, hiddenMockTheme, (ScrollbarThemeOverlayMock::HideScrollbars));
+            return hiddenMockTheme;
+        }
         if (RuntimeEnabledFeatures::overlayScrollbarsEnabled()) {
             DEFINE_STATIC_LOCAL(ScrollbarThemeOverlayMock, overlayMockTheme, ());
             return overlayMockTheme;
@@ -304,6 +308,10 @@ ScrollbarTheme& ScrollbarTheme::theme()
         DEFINE_STATIC_LOCAL(ScrollbarThemeMock, mockTheme, ());
         return mockTheme;
     }
+    if (RuntimeEnabledFeatures::overlayScrollbarsEnabled() && RuntimeEnabledFeatures::hideScrollbarsEnabled()) {

[skobes, 2016/08/12 18:29:54]

Do we need to check overlayScrollbarsEnabled(), or is it implied by
hideScrollbarsEnabled()?

+        DEFINE_STATIC_LOCAL(ScrollbarThemeOverlay, hiddenTheme, (0, 0, ScrollbarThemeOverlay::DisallowHitTest, Color()));
+        return hiddenTheme;
+    }
     return nativeTheme();
 }