Add TODOs for broken build code in aggregate_vector_icons.

ui/gfx/BUILD.gn

Index: ui/gfx/BUILD.gn
diff --git a/ui/gfx/BUILD.gn b/ui/gfx/BUILD.gn
index a3cc04f36ed0c3c8836e9800f0d79a0cfa98b3e0..3bb45dd62f6abfeac0a05fc97934dd3a1c9bc23c 100644
--- a/ui/gfx/BUILD.gn
+++ b/ui/gfx/BUILD.gn
@@ -569,6 +569,11 @@ action("aggregate_vector_icons") {
   ]
 
   args = [
+    # TODO(brettw) bug 535386: This should not take a directory as an input,
+    # but rather a response file listing the inputs or sometimes the build will
+    # be incorrect. In this case, Ninja won't be able to do proper dependency
+    # tracking since if a file is deleted, the command line will be the same
+    # and the action will not be re-run.
     "--working_directory=" + rebase_path("vector_icons/"),
     "--output_cc=" + rebase_path(vector_icons_cc_file, root_build_dir),
     "--output_h=" + rebase_path(vector_icons_h_file, root_build_dir),