remove unneeded forward declaration

This class no longer uses anything in this file.

BUG=none
TEST=none

Change-Id: I82af529c8fb5768bb2cf2108a4e5aa8ddd848104
Reviewed-on: https://chromium-review.googlesource.com/1176394Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583464}

base/threading/thread_restrictions.h

@@ -127,10 +127,6 @@ namespace ui {
 class WindowResizeHelperMac;
 }
 
-namespace views {
-class ScreenMus;
-}
-
 namespace viz {
 class HostGpuMemoryBufferManager;
 }
@@ -506,7 +502,6 @@ class BASE_EXPORT ThreadRestrictions {
 #if !defined(OFFICIAL_BUILD)
   friend class content::SoftwareOutputDeviceMus;  // Interim non-production code
 #endif
-  friend class views::ScreenMus;
 // END USAGE THAT NEEDS TO BE FIXED.
 
 #if DCHECK_IS_ON()