[GN Closure Compile] Fix issue with externs changes not rebuilding.

This CL fixes an issue where closure compilations may not rebuild if
only their externs files change.

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ie18c5e658742e4025658d082477a9ab87291e065
Reviewed-on: https://chromium-review.googlesource.com/1096809Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566654}

third_party/closure_compiler/compile_js.gni

@@ -69,6 +69,7 @@ template("js_library") {
     args += [ "--sources" ] + rebase_path(sources, root_build_dir)
     if (defined(extra_sources)) {
       args += rebase_path(extra_sources, root_build_dir)
+      sources += extra_sources
     }
 
     if (defined(deps)) {
@@ -83,6 +84,7 @@ template("js_library") {
     }
     if (defined(externs_list)) {
       args += [ "--externs" ] + rebase_path(externs_list, root_build_dir)
+      sources += externs_list
     }
     if (defined(extra_deps)) {
       if (!defined(deps)) {