Re-enable precompiled headers in GN on Windows.

Re-enable precompiled headers in GN on Windows.

This fixes the incremental build error by disabling the warning (see the code for a detailed description).

It adds conditions for goma and official builds which match GYP.

Committed: https://crrev.com/48d359b13c0f60e444280afc03df8dfeb8ebe41d
Cr-Commit-Position: refs/heads/master@{#340769}

[brettw, 2015-07-28 19:56:42]

brettw@chromium.org changed reviewers:
+ dpranke@chromium.org

[brettw, 2015-07-28 19:56:48]

The CQ bit was checked by brettw@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-28 19:57:56]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261953002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261953002/1

[Dirk Pranke, 2015-07-28 20:38:17]

lgtm

[brettw, 2015-07-28 20:46:14]

The CQ bit was checked by brettw@chromium.org

[brettw, 2015-07-28 20:46:14]

The patchset sent to the CQ was uploaded after l-g-t-m from dpranke@chromium.org
Link to the patchset: https://codereview.chromium.org/1261953002/#ps20001
(title: "more flags")

[commit-bot: I haz the power, 2015-07-28 20:47:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261953002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261953002/20001

[commit-bot: I haz the power, 2015-07-28 21:26:55]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-28 21:27:31]

Patchset 2 (id:??) landed as
https://crrev.com/48d359b13c0f60e444280afc03df8dfeb8ebe41d
Cr-Commit-Position: refs/heads/master@{#340769}