Fix 'gn clean'.

Fix 'gn clean'.

The new 'gn clean' command didn't actually work right; when we
read in the existing build.ninja file, we would trim the leading
whitespace from the lines, producing an invalide ninja file as a result.

R=brettw@chromium.org
BUG=

Committed: https://crrev.com/5c1a294cd30d44560ce45980a809f86146584761
Cr-Commit-Position: refs/heads/master@{#318189}

[brettw, 2015-02-24 23:59:34]

lgtm

[tfarina, 2015-02-25 00:04:23]

tfarina@chromium.org changed reviewers:
+ tfarina@chromium.org

[tfarina, 2015-02-25 00:04:23]

lgtm

[Dirk Pranke, 2015-02-26 03:28:55]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-26 03:29:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/952053004/1

[commit-bot: I haz the power, 2015-02-26 05:07:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-26 05:08:10]

Patchset 1 (id:??) landed as
https://crrev.com/5c1a294cd30d44560ce45980a809f86146584761
Cr-Commit-Position: refs/heads/master@{#318189}