[ChromeDriver] Support download.default_directory for Headless

This CL enables users in headless mode to specify download.default_directory using
dot notation rather than constructing a nested dictionary. Support for
the nested syntax is maintained. This matches the behavior for headed mode.

Bug: chromedriver:3106
Change-Id: I8b225b83272b5ab15d176603bf6df43493c64a4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779427Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tricia Crichton <triciac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692278}

chrome/test/chromedriver/session_commands.cc

@@ -317,8 +317,10 @@ Status InitSessionHelper(const InitSessionParams& bound_params,
   if (session->chrome->GetBrowserInfo()->is_headless) {
     std::string download_directory;
     if (capabilities.prefs &&
-        capabilities.prefs->GetString("download.default_directory",
-                                      &download_directory))
+        (capabilities.prefs->GetString("download.default_directory",
+                                       &download_directory) ||
+         capabilities.prefs->GetStringWithoutPathExpansion(
+             "download.default_directory", &download_directory)))
       session->headless_download_directory =
           std::make_unique<std::string>(download_directory);
     else