[headless] Change --headless switch default to 'new'

Specifying the --headless switch will activate new Headless mode.
Old Headless mode is still available if requested explicitly with
--headless=old.

Bug: 346606149
Change-Id: I1ec3ce8ced0a602f66355735d5d282d5dd3cea77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5226762
Auto-Submit: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316031}

chrome/browser/headless/headless_mode_util.cc

diff --git a/chrome/browser/headless/headless_mode_util.cc b/chrome/browser/headless/headless_mode_util.cc
index fdfeca2..ef813e12 100644
--- a/chrome/browser/headless/headless_mode_util.cc
+++ b/chrome/browser/headless/headless_mode_util.cc
@@ -34,7 +34,7 @@
   kNoHeadlessMode,
   kOldHeadlessMode,
   kNewHeadlessMode,
-  kDefaultHeadlessMode = kOldHeadlessMode
+  kDefaultHeadlessMode = kNewHeadlessMode
 };
 
 HeadlessMode GetHeadlessMode() {