Remove default value checking in GN, adds getenv function, reorders parameters to rebase_path.

tools/gn/args.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/args.h"
 
 #include "build/build_config.h"
 #include "tools/gn/variables.h"
 
 #if defined(OS_WIN)
@@ skipping 97 matching lines @@
                        Err* err) const {
   base::AutoLock lock(lock_);
 
   for (Scope::KeyValueMap::const_iterator i = args.begin();
        i != args.end(); ++i) {
     // Verify that the value hasn't already been declared. We want each value
     // to be declared only once.
     //
     // The tricky part is that a buildfile can be interpreted multiple times
     // when used from different toolchains, so we can't just check that we've
-    // seen it before. Instead, we check that the location matches. We
-    // additionally check that the value matches to prevent people from
-    // declaring defaults based on other parameters that may change. The
-    // rationale is that you should have exactly one default value for each
-    // argument that we can display in the help.
+    // seen it before. Instead, we check that the location matches.
     Scope::KeyValueMap::iterator previously_declared =
         declared_arguments_.find(i->first);
     if (previously_declared != declared_arguments_.end()) {
       if (previously_declared->second.origin() != i->second.origin()) {
         // Declaration location mismatch.
         *err = Err(i->second.origin(), "Duplicate build argument declaration.",
             "Here you're declaring an argument that was already declared "
             "elsewhere.\nYou can only declare each argument once in the entire "
             "build so there is one\ncanonical place for documentation and the "
             "default value. Either move this\nargument to the build config "
             "file (for visibility everywhere) or to a .gni file\nthat you "
             "\"import\" from the files where you need it (preferred).");
         err->AppendSubErr(Err(previously_declared->second.origin(),
                               "Previous declaration.",
                               "See also \"gn help buildargs\" for more on how "
                               "build arguments work."));
         return false;
-      } else if (previously_declared->second != i->second) {
-        // Default value mismatch.
-        *err = Err(i->second.origin(),
-            "Non-constant default value for build argument.",
-            "Each build argument should have one default value so we report "
-            "it nicely in the\n\"gn args\" command. Please make this value "
-            "constant.");
-        return false;
       }
     } else {
       declared_arguments_.insert(*i);
     }
 
     // Only set on the current scope to the new value if it hasn't been already
     // set. Mark the variable used so the build script can override it in
     // certain cases without getting unused value errors.
     if (!scope_to_set->GetValue(i->first)) {
       scope_to_set->SetValue(i->first, i->second, i->second.origin());
@@ skipping 109 matching lines @@
   for (Scope::KeyValueMap::const_iterator i = values.begin();
        i != values.end(); ++i)
     scope->SetValue(i->first, i->second, i->second.origin());
 }
 
 void Args::SaveOverrideRecordLocked(const Scope::KeyValueMap& values) const {
   for (Scope::KeyValueMap::const_iterator i = values.begin();
        i != values.end(); ++i)
     all_overrides_[i->first] = i->second;
 }