gn/gyp: Escape \ in addition to $ and " in strings.

BUG=340055
R=halyavin@google.com
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/144573004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@248410 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

gyp_chromium

@@ -77,8 +77,9 @@ def FormatKeyForGN(key):
 
 def EscapeStringForGN(s):
   """Converts a string to a GN string literal."""
-  # Escape $ characters which have special meaning to GN.
-  return '"' + s.replace('$', '\\$').replace('"', '\\"') + '"'
+  for old, new in [('\\', '\\\\'), ('$', '\\$'), ('"', '\\"')]:
+    s = s.replace(old, new)
+  return '"' + s + '"'
 
 
 def ProcessGypDefinesItems(items):