Relanding of Add policy_templates target for GN.

Relanding Add policy_templates target for GN. (patchset #1 id:1 of https://codereview.chromium.org/992863002/)

Landed as a part of issue 997523002

Reason for revert:
This did not break the build.

Original issue's description:
> Revert of Add policy_templates target for GN. (patchset #4 id:60001 of https://codereview.chromium.org/957283003/)
> 
> Reason for revert:
> Possibly broke some GN builds (reverted with https://codereview.chromium.org/865573002), e.g.:
> 
> http://build.chromium.org/p/chromium.linux/builders/Linux%20GN/builds/24483/steps/compile/logs/stdio
> http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20GN/builds/11092/steps/compile/logs/stdio
> 
> ninja: Entering directory `/mnt/data/b/build/slave/Linux_GN/build/src/out/Release'
> ninja:error: expected depfile 'gen/chrome/policy_templates.d' to mention 'gen/chrome/app/policy/common/html/am/chrome_policy_list.html', got 'gen/chrome/app/policy/linux/examples/chrome.json'
> 
> Original issue's description:
> > Add policy_templates target for GN.
> >
> > BUG=462362
> >
> > Committed: https://crrev.com/982eb90bf74535cbe5e21c4eacb759c42ff424d4
> > Cr-Commit-Position: refs/heads/master@{#319759}
> 
> TBR=pneubeck@chromium.org,bauerb@chromium.org,knn@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=462362
> 
> Committed: https://crrev.com/a164bcfcacb9fdf84bf73a0b98042424aada3b55
> Cr-Commit-Position: refs/heads/master@{#319774}

TBR=pneubeck@chromium.org,bauerb@chromium.org,dgrogan@chromium.org
BUG=462362

[knn, 2015-03-10 11:34:16]

Created Revert of Revert of Add policy_templates target for GN.