Revert "gen_amalgamated: append to default GN args"

This reverts commit 902a8a62c06ad0858f43af527c4e2d9ade3e7b89.

Reason for revert: is_perfetto_build_generator=true creates a static library per perfetto_component() GN template call and doesn't work well for CrOS, in using libperfetto_client_experimental.a as the SDK static library libperfeto_sdk.a.

Change-Id: Id209806c55e3d306503ea2169de60782696f528b
diff --git a/tools/gen_amalgamated b/tools/gen_amalgamated
index 1613ebe..c529155 100755
--- a/tools/gen_amalgamated
+++ b/tools/gen_amalgamated
@@ -42,7 +42,7 @@
 
 # Arguments for the GN output directory (unless overridden from the command
 # line).
-default_gn_args = ' '.join([
+gn_args = ' '.join([
     'enable_perfetto_ipc=true',
     'is_debug=false',
     'is_perfetto_build_generator=true',
@@ -558,9 +558,8 @@
       default=os.path.join(gn_utils.repo_root(), 'out/amalgamated/perfetto'))
   parser.add_argument(
       '--gn_args',
-      help='GN arguments used to prepare the output directory in addition to '
-           'the default arguments',
-      default='')
+      help='GN arguments used to prepare the output directory',
+      default=gn_args)
   parser.add_argument(
       '--keep',
       help='Don\'t delete the GN output directory at exit',
@@ -585,7 +584,6 @@
       help='Targets to include in the output (e.g., "//:libperfetto")')
   args = parser.parse_args()
   targets = args.targets or default_targets
-  gn_args = ' '.join([default_gn_args, args.gn_args])
 
   # The CHANGELOG mtime triggers the perfetto_version.gen.h genrule. This is
   # to avoid emitting a stale version information in the remote case of somebody
@@ -598,7 +596,7 @@
   if args.check:
     output = os.path.join(tempfile.mkdtemp(), 'perfetto_amalgamated')
 
-  out = gn_utils.prepare_out_directory(gn_args,
+  out = gn_utils.prepare_out_directory(args.gn_args,
                                        args.out,
                                        system_buildtools=args.system_buildtools)
   if not args.quiet: