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 5a59eac783a44911681a4958d5505a5076f33533..208cbbf1e9421724891c32605f12cbe882661632 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",
@@ -244,10 +246,14 @@ static_library("headless_lib") {
     "//ui/base",
     "//ui/compositor",
     "//ui/display",
-    "//ui/ozone",
+    "//ui/events/devices",
     "//url",
   ]
 
+  if (use_ozone) {
+    deps += [ "//ui/ozone" ]
+  }
+
   configs += [ ":headless_implementation" ]
 }