Revert of Revert of tools/gn: fix escaping of backslashes (patchset #1 id:1 of https://codereview.chromium.org/1049963002/)

Reason for revert:
Reland as it can't be the cause of the issue.

Original issue's description:
> Revert of tools/gn: fix escaping of backslashes (patchset #3 id:40001 of https://codereview.chromium.org/995393005/)
>
> Reason for revert:
> Speculatively reverting because of: http://build.chromium.org/p/chromium.win/buildstatus?builder=Win8%20GN%20%28dbg%29&number=4637
>
> ERROR at //third_party/WebKit/Source/bindings/scripts/scripts.gni:198:5: Unable to write file.
>     write_file(idl_files_list, rebase_path(invoker.sources, root_build_dir))
>
> Original issue's description:
> > tools/gn: fix escaping of backslashes
> >
> > BUG=470217
> >
> > Committed: https://crrev.com/4bf46ca929d17910fcf66cc25e886b986845ca59
> > Cr-Commit-Position: refs/heads/master@{#323161}
>
> TBR=brettw@chromium.org,mdempsky@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=470217
>
> Committed: https://crrev.com/745f645d044cede33a60e03454bbf797df627cce
> Cr-Commit-Position: refs/heads/master@{#323167}

TBR=brettw@chromium.org,mdempsky@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=470217

Review URL: https://codereview.chromium.org/1043383002

Cr-Commit-Position: refs/heads/master@{#323168}

1 files changed, 21 insertions, 9 deletions

diff --git a/tools/gn/value.cc b/tools/gn/value.cc
index 6dfe234..1a4215c 100644
--- a/tools/gn/value.cc
+++ b/tools/gn/value.cc
@@ -124,15 +124,27 @@ std::string Value::ToString(bool quote_string) const {
       return base::Int64ToString(int_value_);
     case STRING:
       if (quote_string) {
-        std::string escaped = string_value_;
-        // First escape all special uses of a backslash.
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\\$", "\\\\$");
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\\\"", "\\\\\"");
-
-        // Now escape special chars.
-        ReplaceSubstringsAfterOffset(&escaped, 0, "$", "\\$");
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\"", "\\\"");
-        return "\"" + escaped + "\"";
+        std::string result = "\"";
+        bool hanging_backslash = false;
+        for (char ch : string_value_) {
+          // If the last character was a literal backslash and the next
+          // character could form a valid escape sequence, we need to insert
+          // an extra backslash to prevent that.
+          if (hanging_backslash && (ch == '$' || ch == '"' || ch == '\\'))
+            result += '\\';
+          // If the next character is a dollar sign or double quote, it needs
+          // to be escaped; otherwise it can be printed as is.
+          if (ch == '$' || ch == '"')
+            result += '\\';
+          result += ch;
+          hanging_backslash = (ch == '\\');
+        }
+        // Again, we need to prevent the closing double quotes from becoming
+        // an escape sequence.
+        if (hanging_backslash)
+          result += '\\';
+        result += '"';
+        return result;
       }
       return string_value_;
     case LIST: {