Don't add implicit deps to copy commands in GN.

tools/gn/function_toolchain.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scheduler.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/settings.h"
@@ skipping 141 matching lines @@
 const char kTool_HelpShort[] =
     "tool: Specify arguments to a toolchain tool.";
 const char kTool_Help[] =
     "tool: Specify arguments to a toolchain tool.\n"
     "\n"
     "  tool(<command type>) { <command flags> }\n"
     "\n"
     "  Used inside a toolchain definition to define a command to run for a\n"
     "  given file type. See also \"gn help toolchain\".\n"
     "\n"
-    "Command types:\n"
+    "Command types\n"
     "\n"
     "  The following values may be passed to the tool() function for the type\n"
     "  of the command:\n"
     "\n"
     "  \"cc\", \"cxx\", \"objc\", \"objcxx\", \"asm\", \"alink\", \"solink\",\n"
     "  \"link\", \"stamp\", \"copy\"\n"
     "\n"
+    "Tool-specific notes\n"
+    "\n"
+    "  copy\n"
+    "    The copy command should be a native OS command since it does not\n"
+    "    implement toolchain dependencies (which would enable a copy tool to\n"
+    "    be compiled by a previous step).\n"
+    "\n"
+    "    It is legal for the copy to not update the timestamp of the output\n"
+    "    file (as long as it's greater than or equal to the input file). This\n"
+    "    allows the copy command to be implemented as a hard link which can\n"
+    "    be more efficient.\n"
+    "\n"
     "Command flags\n"
     "\n"
     "  These variables may be specified in the { } block after the tool call.\n"
     "  They are passed directly to Ninja. See the ninja documentation for how\n"
     "  they work. Don't forget to backslash-escape $ required by Ninja to\n"
     "  prevent GN from doing variable expansion.\n"
     "\n"
     "    command, depfile, depsformat, description, pool, restat, rspfile,\n"
     "    rspfile_content\n"
     "\n"
@@ skipping 182 matching lines @@
     return Value();
 
   Scope::KeyValueMap values;
   block_scope.GetCurrentScopeValues(&values);
   toolchain->args() = values;
 
   return Value();
 }
 
 }  // namespace functions