Disable overlay scrollbars to hide them on non-Mac.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
index 86eec510281cc39bc1b4747f0a84ab7d84f9e8f9..f93976a7f244469f92934f174e3588b8ca618dfa 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -71,6 +71,13 @@ class PLATFORM_EXPORT ScrollbarTheme {
   virtual bool usesOverlayScrollbars() const { return false; }
   virtual void updateScrollbarOverlayColorTheme(const ScrollbarThemeClient&) {}
 
+  // If true, scrollbars that become invisible (i.e. overlay scrollbars that
+  // fade out) should be marked as disabled. This option exists since Mac and
+  // Aura overlays implement the fade out differently, with Mac painting code
+  // fading out the scrollbars. Aura scrollbars require disabling the scrollbar
+  // to prevent painting it.
+  virtual bool disableInvisibleScrollbars() const { return true; }

[jbroman, 2016/11/18 19:00:27]

nit: this name suggests that calling it disables the scrollbars. Could it be
named "disablesInvisibleScrollbars()" (like usesOverlayScrollbars above), or
"shouldDisableInvisibleScrollbars()", or something along those lines?

[bokan, 2016/11/18 19:09:00]

Good point, done.

+
   virtual bool invalidateOnMouseEnterExit() { return false; }
   virtual bool invalidateOnWindowActiveChange() const { return false; }