Absolute path fixes for gn.

tools/gn/label_pattern.cc

Index: tools/gn/label_pattern.cc
diff --git a/tools/gn/label_pattern.cc b/tools/gn/label_pattern.cc
index 396a9b73a9d4fc1f269d926496b2dda5d0a46e89..4b31174d946dfb2f64f8834f31e00b2b1dc3e1b5 100644
--- a/tools/gn/label_pattern.cc
+++ b/tools/gn/label_pattern.cc
@@ -4,6 +4,7 @@
 
 #include "tools/gn/label_pattern.h"
 
+#include "base/strings/string_util.h"
 #include "tools/gn/err.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/value.h"
@@ -121,7 +122,23 @@ LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
   // Extract path and name.
   base::StringPiece path;
   base::StringPiece name;
-  size_t colon = str.find(':');
+  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]) &&
+        IsAsciiAlpha(str[1])) {
+      // Skip over the drive letter colon.
+      offset = 3;
+    }
+  }
+#endif
+  size_t colon = str.find(':', offset);
   if (colon == std::string::npos) {
     path = base::StringPiece(str);
   } else {