Accept absolute Windows paths without leading slash in GN commands

tools/gn/label_unittest.cc

Index: tools/gn/label_unittest.cc
diff --git a/tools/gn/label_unittest.cc b/tools/gn/label_unittest.cc
index 37c097f23224e52e92598ab9eae44907a84ab5d2..f4bfc07a1543c67aa4a9da9f79621aab988f1d26 100644
--- a/tools/gn/label_unittest.cc
+++ b/tools/gn/label_unittest.cc
@@ -40,7 +40,7 @@ TEST(Label, Resolve) {
 #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" },