Revert of Revert of Sync Windows build flags between GYP and GN

Revert of Revert of Sync Windows build flags between GYP and GN (patchset #1 id:1 of https://codereview.chromium.org/1185963004/)

Reason for revert:
Appears this was innocent after all.

Original issue's description:
> Revert of Sync Windows build flags between GYP and GN (patchset #4 id:60001 of https://codereview.chromium.org/1183613006/)
>
> Reason for revert:
> Speculative revert for Linux GN clobber compile failure:
>
> FAILED: python ../../remoting/webapp/build-webapp.py ...
> Traceback (most recent call last):
>   File "../../remoting/webapp/build-webapp.py", line 487, in <module>
>     sys.exit(main())
>   File "../../remoting/webapp/build-webapp.py", line 483, in main
>     return buildWebApp(**vars(args))
>   File "../../remoting/webapp/build-webapp.py", line 257, in buildWebApp
>     + buildtype + ': ' + service_environment)
> Exception: Invalid service_environment targeted for Release:
> ninja: build stopped: subcommand failed.
>
> Original issue's description:
> > Sync Windows build flags between GYP and GN
> >
> > In GN, turn on omtimize:max for base targets on all platforms, not just Android (matches GYP).
> >
> > In GN, don't duplicate /Os twice, make standard optimize = /O1 (matches GYP). Don't specify redundant optimization flags that are included as part of /O1 or /O2 (doesn't match GYP flag-wise, but should match functionality-wise).
> >
> > In GN, add a min Windows version of XP to x86 architecture binaries (matches GYP).
> >
> > In GYP, remove /GT ("EnableFiberSafeOptimizations"). This flag is super old (pre-dates GYP) and was likely turned on because it has the word "optimization" in it. It was only specified in official builds. However, according to the docs, this is making optimizations fiber safe (i.e. slower, by not caching the TLS pointer), rather than enabling optimizations for fibers. We don't use fibers so this flag is unnecessary and may be making TLS access slightly slower.
> >
> > Committed: https://crrev.com/67021f4d70fe838a5d78133bc6d5b305dfd70d72
> > Cr-Commit-Position: refs/heads/master@{#334742}
>
> TBR=scottmg@chromium.org,brettw@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/1becf72a2d1b5eec4468cc950142b083e9b24fbe
> Cr-Commit-Position: refs/heads/master@{#334782}

TBR=scottmg@chromium.org,brettw@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/6a18ffd1f1e656bba0d905c30fad6b46ec4ffcff
Cr-Commit-Position: refs/heads/master@{#334795}

[phoglund_chromium, 2015-06-17 09:06:15]

Created Revert of Revert of Sync Windows build flags between GYP and GN

[commit-bot: I haz the power, 2015-06-17 09:07:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1184953004/1

[commit-bot: I haz the power, 2015-06-17 09:08:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-17 09:09:52]

Patchset 1 (id:??) landed as
https://crrev.com/6a18ffd1f1e656bba0d905c30fad6b46ec4ffcff
Cr-Commit-Position: refs/heads/master@{#334795}