Build non-official app_shell installer on Linux

This causes the app_shell .deb installer to be built on non-ASAN/MSAN
Linux CQ and waterfall bots, so regressions that break the installer
build can be caught before hitting the official builders.

The Chrome installer was enabled for non-official Linux builders in
https://bugs.chromium.org/p/chromium/issues/detail?id=758654#c32.

Bug: 714465
Change-Id: Ifb6908b756d9ffc292fc131396c424d66f841a2a
Reviewed-on: https://chromium-review.googlesource.com/956756Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542382}

extensions/shell/installer/BUILD.gn

@@ -10,9 +10,8 @@ import("//extensions/buildflags/buildflags.gni")
 assert(enable_extensions)
 
 declare_args() {
-  enable_app_shell_installer =
-      is_desktop_linux && is_chrome_branded && current_cpu == "x64" &&
-      !is_component_build && !using_sanitizer
+  enable_app_shell_installer = is_desktop_linux && current_cpu == "x64" &&
+                               !is_component_build && !using_sanitizer
 }
 
 # Meta-target that forwards to the installer of the correct type (if any).