Enable hidpi on Linux, refactor a bit on Windows to share

ui/gfx/gfx.gyp

Index: ui/gfx/gfx.gyp
diff --git a/ui/gfx/gfx.gyp b/ui/gfx/gfx.gyp
index b385459122099ef0bd7515a0eef2c65ec92bc270..f6d985a1c17bb46f99aac12dd1f2f3a354ace913 100644
--- a/ui/gfx/gfx.gyp
+++ b/ui/gfx/gfx.gyp
@@ -158,6 +158,8 @@
         'display_change_notifier.h',
         'display_observer.cc',
         'display_observer.h',
+        'dpi.cc',
+        'dpi.h',
         'favicon_size.cc',
         'favicon_size.h',
         'font.cc',
@@ -301,8 +303,6 @@
         'vsync_provider.h',
         'win/direct_write.cc',
         'win/direct_write.h',
-        'win/dpi.cc',
-        'win/dpi.h',
         'win/hwnd_util.cc',
         'win/hwnd_util.h',
         'win/scoped_set_map_mode.h',
@@ -418,6 +418,11 @@
             '<(DEPTH)/build/linux/system.gyp:fontconfig',
           ],
         }],
+        ['desktop_linux!=1 and OS!="win"', {
+          'sources!': [
+            'dpi.cc',
+          ],
+        }],
       ],
       'target_conditions': [
         # Need 'target_conditions' to override default filename_rules to include