Also check jumbo_excluded_sources when jumbo is disabled

Since jumbo is still disabled by default and there is no active
bot that checks things, it is best to check that jumbo_excluded_sources
are correct also when jumbo is disabled.

Bug: 713137
Change-Id: I85bb92f283be240a704ce3ec0d5765933ba52016
Reviewed-on: https://chromium-review.googlesource.com/563683
Commit-Queue: bratell at Opera <bratell@opera.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485913}

build/config/jumbo.gni

@@ -133,6 +133,11 @@ template("jumbo_target") {
       args = [ "--outputs" ] + rebase_path(outputs, root_build_dir) +
              [ "--file-list={{response_file_name}}" ]
     }
+  } else {
+    # If the list subtraction triggers a gn error,
+    # jumbo_excluded_sources lists a file that is not in sources.
+    sources_after_exclusion = invoker.sources - excluded_sources
+    assert(sources_after_exclusion != [] || true)  # Prevent "unused variable".
   }
 
   target_type = invoker.target_type