cros: Add VIEWS_EXPORT to MenuSeparator

MenuSeparator is being reused in Ash, but to do this VIEWS_EXPORT
needs to be added to views::MenuSeparator. Else we have linker failures:

std::__1::allocator<char> > const&): error: undefined reference to
'vtable for views::MenuSeparator'
../../third_party/binutils/Linux_x64/Release/bin/ld.gold: the vtable
symbol may be undefined because the class is missing its key function

Bug: 861885
Change-Id: I787fe6bf1d62518d03e52007b792481251ae4174
Reviewed-on: https://chromium-review.googlesource.com/1129999Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573477}

ui/views/controls/menu/menu_separator.h

@@ -9,10 +9,11 @@
 #include "base/macros.h"
 #include "ui/base/models/menu_separator_types.h"
 #include "ui/views/view.h"
+#include "ui/views/views_export.h"
 
 namespace views {
 
-class MenuSeparator : public View {
+class VIEWS_EXPORT MenuSeparator : public View {
  public:
   explicit MenuSeparator(ui::MenuSeparatorType type) : type_(type) {}