Include headless resources in Mac OS

Headless is enable in Mac and Linux, and will soon be enabled in Windows.
This patch adds headless resources (1.7 Kb) to all those versions
Launch bug: http://crbug.com/705916

BUG=709836

Review-Url: https://codereview.chromium.org/2811633002
Cr-Commit-Position: refs/heads/master@{#464299}

chrome/chrome_paks.gni

@@ -157,7 +157,7 @@ template("chrome_extra_paks") {
         "//ui/file_manager:resources",
       ]
     }
-    if (is_linux) {
+    if (is_linux || is_mac) {
       sources += [ "$root_gen_dir/headless/headless_lib_resources.pak" ]
       deps += [ "//headless:resources" ]
     }