Use /etc/brave/policies even if it doesn't exist.

PathService::Override() requires that the new target path exists,
and tries to create it if it doesn't. Since the user doesn't have
permission to create `/etc/brave` the override fails if an
administrator hadn't already installed something there, and the
original `/etc/chromium` path for policy files remains active.

Current uses of this path are few and seem to be robust against
the directory not existing, so this seems a safer fix than patching
the upstream default.

Closes brave/brave-browser#19052

app/brave_main_delegate.cc

@@ -165,9 +165,9 @@ void BraveMainDelegate::PreSandboxStartup() {
 #endif  // defined(OS_LINUX) || defined(OS_MAC)
 
 #if defined(OS_POSIX) && !defined(OS_MAC)
-  base::PathService::Override(
+  base::PathService::OverrideAndCreateIfNeeded(
       chrome::DIR_POLICY_FILES,
-      base::FilePath(FILE_PATH_LITERAL("/etc/brave/policies")));
+      base::FilePath(FILE_PATH_LITERAL("/etc/brave/policies")), true, false);
 #endif
 
   if (brave::SubprocessNeedsResourceBundle()) {