Add UX experimental feature flash scrollbars when mouse enter

This patch introduce an UX experimental feature flash scrollbars when mouse
enter for overlay scrollbar. This feature is behind a flag:
OverlayScrollbarFlashWhenMouseEnter.

Bug: 746342
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I72bf4ce1538af3af356723fbde7d2828676607d3
Reviewed-on: https://chromium-review.googlesource.com/627298
Commit-Queue: Jianpeng Chao <[email protected]>
Reviewed-by: weiliangc <[email protected]>
Reviewed-by: Philip Rogers <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Reviewed-by: Evan Stade <[email protected]>
Cr-Commit-Position: refs/heads/master@{#497142}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 0ec5297..d9b54f6 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -936,6 +936,12 @@
     "Flash Overlay Scrollbars After any scroll update happends in page. You"
     " must also enable Overlay Scrollbars.";
 
+const char kOverlayScrollbarsFlashWhenMouseEnterName[] =
+    "Flash Overlay Scrollbars When Mouse Enter";
+const char kOverlayScrollbarsFlashWhenMouseEnterDescription[] =
+    "Flash Overlay Scrollbars When Mouse Enter a scrollable area. You must also"
+    " enable Overlay Scrollbars.";
+
 const char kOverscrollHistoryNavigationName[] = "Overscroll history navigation";
 const char kOverscrollHistoryNavigationDescription[] =
     "Experimental history navigation in response to horizontal overscroll.";