Use not_needed() rather than asserts to avoid warnings in jumbo.gni

gn has a function not_needed() intended to avoid "Variable is not
used" errors in gni files. So far the code has used asserts for the
same purpose but it is better to use the function intended for the
purpose.

Change-Id: I5ec7e50c4089b265e104aaede24d0e4e50452cef
Reviewed-on: https://chromium-review.googlesource.com/c/1314588Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#605338}

build/config/jumbo.gni

@@ -86,7 +86,7 @@ template("internal_jumbo_target") {
 
   gen_target_dir = invoker.target_gen_dir
 
-  assert(gen_target_dir != "")  # Prevent "unused variable".
+  not_needed([ "gen_target_dir" ])  # Prevent "unused variable".
 
   if (use_jumbo_build_for_target) {
     jumbo_files = []
@@ -173,16 +173,14 @@ template("internal_jumbo_target") {
     # 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".
+    not_needed([ "sources_after_exclusion" ])
   }
 
   target_type = invoker.target_type
   if (use_jumbo_build_for_target && target_type == "split_static_library") {
     # Meaningless and also impossible if split_count > len(jumbo_files)
     target_type = "static_library"
-
-    # Prevent "unused variable" warning.
-    assert(!defined(invoker.split_count) || invoker.split_count > 0)
+    not_needed(invoker, [ "split_count" ])
   }
 
   # Perform the actual operation, either on the original sources or