Extract non-IO part of mksnapshot into an API method.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 694 matching lines @@
 DEFINE_INT(testing_prng_seed, 42, "Seed used for threading test randomness")
 #ifdef _WIN32
 DEFINE_STRING(testing_serialization_file, "C:\\Windows\\Temp\\serdes",
               "file in which to testing_serialize heap")
 #else
 DEFINE_STRING(testing_serialization_file, "/tmp/serdes",
               "file in which to serialize heap")
 #endif
 
 // mksnapshot.cc
-DEFINE_STRING(extra_code, NULL,
-              "A filename with extra code to be included in"
-              " the snapshot (mksnapshot only)")
 DEFINE_STRING(startup_blob, NULL,
-              "Write V8 startup blob file. "
-              "(mksnapshot only)")
+              "Write V8 startup blob file. (mksnapshot only)")
 
 // code-stubs-hydrogen.cc
 DEFINE_BOOL(profile_hydrogen_code_stub_compilation, false,
             "Print the time it takes to lazily compile hydrogen code stubs.")
 
 DEFINE_BOOL(predictable, false, "enable predictable mode")
 DEFINE_NEG_IMPLICATION(predictable, concurrent_recompilation)
 DEFINE_NEG_IMPLICATION(predictable, concurrent_osr)
 DEFINE_NEG_IMPLICATION(predictable, concurrent_sweeping)
 
@@ skipping 267 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA