Revert of tools/gn: fix escaping of backslashes

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}

[raymes, 2015-04-01 02:09:39]

Created Revert of tools/gn: fix escaping of backslashes

[commit-bot: I haz the power, 2015-04-01 02:09:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1049963002/1

[commit-bot: I haz the power, 2015-04-01 02:10:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-01 02:11:23]

Patchset 1 (id:??) landed as
https://crrev.com/745f645d044cede33a60e03454bbf797df627cce
Cr-Commit-Position: refs/heads/master@{#323167}

[mdempsky, 2015-04-01 02:12:44]

GN hasn't been rolled since March 18.

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

In that case, sorry for the noise. I'll re-land it.

[raymes, 2015-04-01 02:13:31]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1043383002/ by raymes@chromium.org.

The reason for reverting is: Reland as it can't be the cause of the issue..

[mdempsky, 2015-04-01 02:18:12]

On 2015/04/01 02:13:13, raymes wrote:
> In that case, sorry for the noise. I'll re-land it.

Thanks.