Remove unnecessary libXss dependency via //build/config/linux:x11

It's unnecessary to add the libXss dependency for any target that
depends on //build/config/linux:x11. Instead, the libXss dependency
should be pulled in explicitly via //build/config/linux:xscrnsaver.
This change makes the GN flow more in line with the GYP flow, especially
when building for Chrome OS.

BUG=427726
TEST=Manually test Linux build.
TEST=Manually verify that the libXss dependency is not added when building for Chrome OS with 'os="chromeos"'

Review URL: https://codereview.chromium.org/682893003

Cr-Original-Commit-Position: refs/heads/master@{#301802}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 407ead664abeab9534ceee416ef637de13c4e247

config/linux/BUILD.gn

@@ -68,8 +68,6 @@ if (use_evdev_gestures) {
 }
 
 config("x11") {
-  # Don't bother running pkg-config for these X related libraries since it just
-  # returns the same libs, and forking pkg-config is slow.
   libs = [
     "X11",
     "Xcomposite",
@@ -79,7 +77,6 @@ config("x11") {
     "Xfixes",
     "Xi",
     "Xrender",
-    "Xss",
     "Xtst",
   ]
 }