Revert of Revert of tools/gn: fix escaping of backslashes

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
Committed: https://crrev.com/714f45ab973ab09d727dfaed319ef62789ae9393
Cr-Commit-Position: refs/heads/master@{#323168}

[raymes, 2015-04-01 02:13:32]

Created Revert of Revert of tools/gn: fix escaping of backslashes

[commit-bot: I haz the power, 2015-04-01 02:14:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1043383002/1

[commit-bot: I haz the power, 2015-04-01 02:14:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-01 02:15:30]

Patchset 1 (id:??) landed as
https://crrev.com/714f45ab973ab09d727dfaed319ef62789ae9393
Cr-Commit-Position: refs/heads/master@{#323168}