Add AppShell symbols to gn_all

Add the app_shell_linux_symbols target to gn_all for official
Linux desktop buliders.

Bug: 818078
Change-Id: I3af605e18fe5fb85ab87f70ee3131d5176de8dde
Reviewed-on: https://chromium-review.googlesource.com/963754Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543774}

BUILD.gn

@@ -481,6 +481,9 @@ group("gn_all") {
         "//extensions/shell:app_shell",
         "//extensions/shell/installer:app_shell_installer",
       ]
+      if (is_desktop_linux && is_official_build) {
+        deps += [ "//extensions/shell:app_shell_linux_symbols" ]
+      }
     }
 
     if (enable_nacl) {