Change GN defaults for toolchains for CrOS.

build/config/sysroot.gni

Index: build/config/sysroot.gni
diff --git a/build/config/sysroot.gni b/build/config/sysroot.gni
index 44ded33e42cf4e4b9eb2fedb920c1eeaa7af72b7..21955c4758b5048c51344bda1920dd98e4064ce4 100644
--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -9,7 +9,10 @@ import("//build/config/chrome_build.gni")
 
 declare_args() {
   # The absolute path of the sysroot that is applied when compiling using
-  # the target toolchain.
+  # the target toolchain. If target_sysroot is set, it will override the
+  # effect of use_sysroot; this allows you to use both target_sysroot
+  # and use_sysroot to set one sysroot for the target and turn off
+  # sysroots for the other toolchains.
   target_sysroot = ""
   use_sysroot = true
 }