Implement a runtime headless mode for Linux

headless/BUILD.gn

Index: headless/BUILD.gn
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index 9c9d392f436b0d91c16679eb3c0cda4d70470a79..705abb66dd791c937abef512520958efbde28bdc 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -175,6 +175,8 @@ static_library("headless_lib") {
               "lib/browser/headless_web_contents_impl.h",
               "lib/browser/headless_window_parenting_client.cc",
               "lib/browser/headless_window_parenting_client.h",
+              "lib/browser/headless_window_tree_host.cc",
+              "lib/browser/headless_window_tree_host.h",
               "lib/headless_content_client.cc",
               "lib/headless_content_client.h",
               "lib/headless_content_main_delegate.cc",
@@ -244,7 +246,7 @@ static_library("headless_lib") {
     "//ui/base",
     "//ui/compositor",
     "//ui/display",
-    "//ui/ozone",
+    "//ui/events/devices",
     "//url",
   ]
 
@@ -349,6 +351,7 @@ test("headless_browsertests") {
 static_library("headless_shell_lib") {
   sources = [
     "app/headless_shell.cc",
+    "app/headless_shell.h",
     "app/headless_shell_switches.cc",
     "app/headless_shell_switches.h",
     "public/headless_shell.h",