linux: Both gtk and the aura port should set the icon theme name.

base/nix/mime_util_xdg.h

Index: base/nix/mime_util_xdg.h
diff --git a/base/nix/mime_util_xdg.h b/base/nix/mime_util_xdg.h
index 79dba89d1dfef1f91113785e2f631656361f9b3e..79eb782e60aa0549ed9d618909e44b397114fef5 100644
--- a/base/nix/mime_util_xdg.h
+++ b/base/nix/mime_util_xdg.h
@@ -25,12 +25,10 @@ BASE_EXPORT std::string GetFileMimeType(const FilePath& filepath);
 // Get the mime type for a byte vector.
 BASE_EXPORT std::string GetDataMimeType(const std::string& data);
 
-#if defined(TOOLKIT_GTK)
-// This detects the current GTK theme by calling gtk_settings_get_default().
-// It should only be executed on the UI thread and must be called before
-// GetMimeIcon().
-BASE_EXPORT void DetectGtkTheme();
-#endif
+// Sets the current icon theme that we've detected from the desktop
+// environment. Currently only works when we believe we're in a GTK
+// environment.
+BASE_EXPORT void SetIconThemeName(const std::string& name);
 
 // Gets the file name for an icon given the mime type and icon pixel size.
 // Where an icon is a square image of |size| x |size|.