Clarify comments about the role of the lex/parse table cache action

Source/bindings/scripts/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/bindings/bindings.gni")
 import("//third_party/WebKit/Source/bindings/scripts/scripts.gni")
 import("//third_party/WebKit/Source/bindings/templates/templates.gni")
 
 visibility = "//third_party/WebKit/*"
 
-# A separate pre-caching step is *not required* to use lex/parse table
-# caching in PLY, as the caches are concurrency-safe.
-# However, pre-caching ensures that all compiler processes use the cached
-# files (hence maximizing speed), instead of early processes building the
-# tables themselves (as they've not yet been written when they start).
+# This separate pre-caching step is required to use lex/parse table
+# caching in PLY, since PLY itself does not check if the cache is
+# valid, and thus may end up using a stale cache if this step hasn't
+# been run to update it.
+#
+# This action's dependencies *is* the cache validation.
 #
 # GYP version: scripts.gyp:cached_lex_yacc_tables
 action("cached_lex_yacc_tables") {
   script = "blink_idl_parser.py"
 
   source_prereqs = idl_lexer_parser_files
   outputs = [
     "$bindings_scripts_output_dir/lextab.py",
     "$bindings_scripts_output_dir/parsetab.pickle",
   ]
@@ skipping 13 matching lines @@
     code_generator_template_files
   # Dummy file to track dependency.
   stamp_file = "$bindings_scripts_output_dir/cached_jinja_templates.stamp"
   outputs = [ stamp_file ]
 
   args = [
     rebase_path(bindings_scripts_output_dir, root_build_dir),
     rebase_path(stamp_file, root_build_dir),
   ]
 }