Fix arm detection code in gn.

Fix arm detection code in gn.

The current code calls substr(3), which gets the substring starting from
index 3 up to the end of the string. However, on Ubuntu armhf (and
likely other systems), arm is the first three characters, and the
original intention was likely to get the substring of length 3 starting
from the beginning of the string.

R=brettw@chromium.org
BUG=607385
Committed: https://crrev.com/b68180a770804afde260b0f1c9747f10139b20b3
Cr-Commit-Position: refs/heads/master@{#390507}

[brettw, 2016-04-28 20:22:21]

lgtm

[saiarcot895, 2016-04-28 20:52:49]

The CQ bit was checked by saiarcot895@gmail.com

[commit-bot: I haz the power, 2016-04-28 20:53:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1920033007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1920033007/1

[commit-bot: I haz the power, 2016-04-28 22:25:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-04-30 17:22:02]

Patchset 1 (id:??) landed as
https://crrev.com/b68180a770804afde260b0f1c9747f10139b20b3
Cr-Commit-Position: refs/heads/master@{#390507}

[commit-bot: I haz the power, 2016-04-30 17:57:58]

Description was changed from

==========
Fix arm detection code in gn.

The current code calls substr(3), which gets the substring starting from
index 3 up to the end of the string. However, on Ubuntu armhf (and
likely other systems), arm is the first three characters, and the
original intention was likely to get the substring of length 3 starting
from the beginning of the string.

R=brettw@chromium.org
BUG=607385
==========

to

==========
Fix arm detection code in gn.

The current code calls substr(3), which gets the substring starting from
index 3 up to the end of the string. However, on Ubuntu armhf (and
likely other systems), arm is the first three characters, and the
original intention was likely to get the substring of length 3 starting
from the beginning of the string.

R=brettw@chromium.org
BUG=607385

Committed: https://crrev.com/b68180a770804afde260b0f1c9747f10139b20b3
Cr-Commit-Position: refs/heads/master@{#390507}
==========