Accept absolute Windows paths without leading slash in GN commands

tools/gn/label_pattern_unittest.cc

Index: tools/gn/label_pattern_unittest.cc
diff --git a/tools/gn/label_pattern_unittest.cc b/tools/gn/label_pattern_unittest.cc
index 19e253027e5517cc469932a182e5d4a08357a6c5..182b7501286743037ef97331ab6ed7ffcfdce721 100644
--- a/tools/gn/label_pattern_unittest.cc
+++ b/tools/gn/label_pattern_unittest.cc
@@ -58,6 +58,15 @@ TEST(LabelPattern, PatternParse) {
     { "//foo(//foo/bar:*)", false, LabelPattern::MATCH, "", "", "" },
     { "//foo/*(*)", false, LabelPattern::MATCH, "", "", "" },
     { "//foo(//bar", false, LabelPattern::MATCH, "", "", "" },
+    // Absolute paths.
+    { "/la/*", true, LabelPattern::RECURSIVE_DIRECTORY, "/la/", "", "" },
+    { "/la:bar", true, LabelPattern::MATCH, "/la/", "bar", "" },
+#if defined(OS_WIN)
+    { "/C:/la/*", true, LabelPattern::RECURSIVE_DIRECTORY, "/C:/la/", "", "" },
+    { "C:/la/*", true, LabelPattern::RECURSIVE_DIRECTORY, "/C:/la/", "", "" },
+    { "/C:/la:bar", true, LabelPattern::MATCH, "/C:/la/", "bar", "" },
+    { "C:/la:bar", true, LabelPattern::MATCH, "/C:/la/", "bar", "" },
+#endif
   };
 
   for (size_t i = 0; i < arraysize(cases); i++) {