Accept absolute Windows paths without leading slash in GN commands

tools/gn/label_pattern.cc

 // 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.
 
 #include "tools/gn/label_pattern.h"
 
 #include <stddef.h>
 
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
@@ skipping 112 matching lines @@
     // Trim off the toolchain for the processing below.
     str = str.substr(0, open_paren);
   }
 
   // Extract path and name.
   base::StringPiece path;
   base::StringPiece name;
   size_t offset = 0;
 #if defined(OS_WIN)
   if (IsPathAbsolute(str)) {
-    if (str[0] != '/') {
-      *err = Err(value, "Bad absolute path.",
-                 "Absolute paths must be of the form /C:\\ but this is \"" +
-                     str.as_string() + "\".");
-      return LabelPattern();
-    }
-    if (str.size() > 3 && str[2] == ':' && IsSlash(str[3]) &&
-        base::IsAsciiAlpha(str[1])) {
+    size_t drive_letter_pos = str[0] == '/' ? 1 : 0;
+    if (str.size() > drive_letter_pos + 2 && str[drive_letter_pos + 1] == ':' &&
+        IsSlash(str[drive_letter_pos + 2]) &&
+        base::IsAsciiAlpha(str[drive_letter_pos])) {
       // Skip over the drive letter colon.
-      offset = 3;
+      offset = drive_letter_pos + 2;
     }
   }
 #endif
   size_t colon = str.find(':', offset);
   if (colon == std::string::npos) {
     path = base::StringPiece(str);
   } else {
     path = str.substr(0, colon);
     name = str.substr(colon + 1);
   }
@@ skipping 109 matching lines @@
       break;
   }
 
   if (!toolchain_.is_null()) {
     result.push_back('(');
     result.append(toolchain_.GetUserVisibleName(false));
     result.push_back(')');
   }
   return result;
 }