Accept absolute Windows paths without leading slash in GN commands

tools/gn/label_unittest.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 <stddef.h>
 
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/err.h"
@@ skipping 22 matching lines @@
       { "//chrome/", ":",                         false, "",                   "",     "",       "" },
       { "//chrome/", "/:",                        false, "",                   "",     "",       "" },
       { "//chrome/", "blah",                      true,  "//chrome/blah/",     "blah", "//t/",   "d" },
       { "//chrome/", "blah:bar",                  true,  "//chrome/blah/",     "bar",  "//t/",   "d" },
       // Absolute paths.
       { "//chrome/", "/chrome:bar",               true , "/chrome/",           "bar",  "//t/",   "d" },
       { "//chrome/", "/chrome/:bar",              true,  "/chrome/",           "bar",  "//t/",   "d" },
 #if defined(OS_WIN)
       { "//chrome/", "/C:/chrome:bar",            true , "/C:/chrome/",        "bar",  "//t/",   "d" },
       { "//chrome/", "/C:/chrome/:bar",           true,  "/C:/chrome/",        "bar",  "//t/",   "d" },
-      { "//chrome/", "C:/chrome:bar",             false, "",                   "",     "",       "" },
+      { "//chrome/", "C:/chrome:bar",             true,  "/C:/chrome/",        "bar",  "//t/",   "d" },

[brettw, 2016/04/05 00:01:57]

Can you add a test here that "C:foo" is parsed as the "foo" target inside the
"C" directory relative to the current one.

 #endif
       // Refers to root dir.
       { "//chrome/", "//:bar",                    true,  "//",                 "bar",  "//t/",   "d" },
       // Implicit directory
       { "//chrome/", ":bar",                      true,  "//chrome/",          "bar",  "//t/",   "d" },
       { "//chrome/renderer/", ":bar",             true,  "//chrome/renderer/", "bar",  "//t/",   "d" },
       // Implicit names.
       { "//chrome/", "//base",                    true,  "//base/",            "base", "//t/",   "d" },
       { "//chrome/", "//base/i18n",               true,  "//base/i18n/",       "i18n", "//t/",   "d" },
       { "//chrome/", "//base/i18n:foo",           true,  "//base/i18n/",       "foo",  "//t/",   "d" },
@@ skipping 33 matching lines @@
       EXPECT_EQ(cur.expected_name, result.name())
           << i << " " << cur.str;
       EXPECT_EQ(cur.expected_toolchain_dir,
                 result.toolchain_dir().value())
           << i << " " << cur.str;
       EXPECT_EQ(cur.expected_toolchain_name, result.toolchain_name())
           << i << " " << cur.str;
     }
   }
 }