Reduce DefaultConcurrentLinks from phys/4GB to phys/5GB.

Reduce DefaultConcurrentLinks from phys/4GB to phys/5GB.

This is copied from the gyp repo, ninja.py:
https://codereview.chromium.org/1255813005

VS 2015 uses about 20% more memory when linking. This caused a
severe page-fault storm on my development machine and subsequent tests
showed that linking could use ~63 GB of RAM (15 parallel links on a
64 GB machine). This suggests that VS 2013 would also be using a lot
of RAM (52.5 GB?) such that very little was left for other
applications or disk cache.

Linker working sets vary wildly but with VS 2013 there are six links
that use over 6 GB of working set and with VS 2015 there are seven.

This change reduces parallel links by 20% (increase RAM-per-linker by
25%). The value can be overridden by setting GYP_LINK_CONCURRENCY.

Tests with VS 2013 showed no statistically significant change in
build times when link concurrency was reduced from 15 to 10 or 12.
Further reductions may be prudent when we switch to VS 2015.

BUG=440500
R=brettw@chromium.org

Committed: https://crrev.com/ad86c6fcef0fbdf26da9165ae8bbbd3748201afc
Cr-Commit-Position: refs/heads/master@{#342892}

[brucedawson, 2015-07-28 18:39:24]

This change was already made in the gyp repo. It appears to improve VS 2013
build times slightly and should avoid page-fault storms with VS 2015.

[brettw, 2015-07-29 20:17:59]

lgtm

[brucedawson, 2015-08-11 20:39:06]

The CQ bit was checked by brucedawson@chromium.org

[commit-bot: I haz the power, 2015-08-11 20:39:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261073004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261073004/1

[commit-bot: I haz the power, 2015-08-11 21:05:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-11 21:06:05]

Patchset 1 (id:??) landed as
https://crrev.com/ad86c6fcef0fbdf26da9165ae8bbbd3748201afc
Cr-Commit-Position: refs/heads/master@{#342892}