Accept absolute Windows paths without leading slash in GN commands

tools/gn/label.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/label.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
 #include "tools/gn/err.h"
@@ skipping 81 matching lines @@
              SourceDir* out_dir,
              std::string* out_name,
              SourceDir* out_toolchain_dir,
              std::string* out_toolchain_name,
              Err* err) {
   // To workaround the problem that StringPiece operator[] doesn't return a ref.
   const char* input_str = input.data();
   size_t offset = 0;
 #if defined(OS_WIN)
   if (IsPathAbsolute(input)) {
-    if (input[0] != '/') {
-      *err = Err(original_value, "Bad absolute path.",
-                 "Absolute paths must be of the form /C:\\ but this is \"" +
-                     input.as_string() + "\".");
-      return false;
-    }
-    if (input.size() > 3 && input[2] == ':' && IsSlash(input[3]) &&
-        base::IsAsciiAlpha(input[1])) {
+    size_t drive_letter_pos = input[0] == '/' ? 1 : 0;
+    if (input.size() > drive_letter_pos + 2 &&
+        input[drive_letter_pos + 1] == ':' &&
+        IsSlash(input[drive_letter_pos + 2]) &&
+        base::IsAsciiAlpha(input[drive_letter_pos])) {
       // Skip over the drive letter colon.
-      offset = 3;
+      offset = drive_letter_pos + 2;
     }
   }
 #endif
   size_t path_separator = input.find_first_of(":(", offset);
   base::StringPiece location_piece;
   base::StringPiece name_piece;
   base::StringPiece toolchain_piece;
   if (path_separator == std::string::npos) {
     location_piece = input;
     // Leave name & toolchain piece null.
@@ skipping 151 matching lines @@
   }
   return ret;
 }
 
 std::string Label::GetUserVisibleName(const Label& default_toolchain) const {
   bool include_toolchain =
       default_toolchain.dir() != toolchain_dir_ ||
       default_toolchain.name() != toolchain_name_;
   return GetUserVisibleName(include_toolchain);
 }