Disable app shell installer when use_sysroot=false

This cl adds an additional condition of use_sysroot=true to enable the app shell
installer.  This matches the Linux condition [1].

This is needed to fix gn bootstrap for Chromium packagers, who build without the
sysroot.

[1] https://cs.chromium.org/chromium/src/chrome/installer/BUILD.gn

BUG=822321

Change-Id: Iae8ba0576525a51a3be91960deb939c760c80b86
Reviewed-on: https://chromium-review.googlesource.com/966967Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543928}

extensions/shell/installer/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
+import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 import("//build/util/process_version.gni")
 import("//extensions/buildflags/buildflags.gni")
@@ -10,8 +11,9 @@ import("//extensions/buildflags/buildflags.gni")
 assert(enable_extensions)
 
 declare_args() {
-  enable_app_shell_installer = is_desktop_linux && current_cpu == "x64" &&
-                               !is_component_build && !using_sanitizer
+  enable_app_shell_installer =
+      is_desktop_linux && current_cpu == "x64" && !is_component_build &&
+      use_sysroot && !using_sanitizer
 }
 
 # Meta-target that forwards to the installer of the correct type (if any).