Fix off-by-one in Array.concat's max index check

Fix off-by-one in Array.concat's max index check

The maximum valid index is strictly smaller than the maximum valid length.

BUG=chromium:516592
LOG=y
R=yangguo@chromium.org

Committed: https://crrev.com/087ae1b08aeda52c8add565b2a53e813a477a6ff
Cr-Commit-Position: refs/heads/master@{#30040}

[Yang, 2015-08-06 09:06:28]

On 2015/08/06 09:06:04, Jakob wrote:

lgtm

[Jakob Kummerow, 2015-08-06 09:06:40]

The CQ bit was checked by jkummerow@chromium.org

[commit-bot: I haz the power, 2015-08-06 09:06:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1278703003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1278703003/1

[commit-bot: I haz the power, 2015-08-06 09:57:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-06 09:57:35]

Patchset 1 (id:??) landed as
https://crrev.com/087ae1b08aeda52c8add565b2a53e813a477a6ff
Cr-Commit-Position: refs/heads/master@{#30040}