fix: Revert "fix: don't propagate GDK_BACKEND to subprocs (#28898)" (#29610)

This reverts commit 7b169c2884.

shell/browser/electron_browser_main_parts.h

@@ -88,11 +88,6 @@ class ElectronBrowserMainParts : public content::BrowserMainParts {
   Browser* browser() { return browser_.get(); }
   BrowserProcessImpl* browser_process() { return fake_browser_process_.get(); }
 
-#if defined(OS_LINUX)
-  // Used by platform_util to set GDK_BACKEND.
-  static base::Optional<std::string>& GetGDKBackend();
-#endif
-
  protected:
   // content::BrowserMainParts:
   int PreEarlyInitialization() override;