Reland of NaCl cleanup: Split out irt_interfaces.cc from irt_ppapi.cc

Reland of NaCl cleanup: Split out irt_interfaces.cc from irt_ppapi.cc (patchset #1 id:1 of https://codereview.chromium.org/1249473003/)

Reason for reland:
The size increase was caused by a Skia DEPS roll (https://codereview.chromium.org/1245793002/), not by this change.

Original issue's description:
> Revert of NaCl cleanup: Split out irt_interfaces.cc from irt_ppapi.cc (patchset #1 id:1 of https://codereview.chromium.org/1244533006/)
>
> Reason for revert:
> Regressed Linux sizes - 4.5% is a bit of a hit to take to bump sizes. If you're absolutely sure this is acceptable, update sizes as well.
>
> Original issue's description:
> > NaCl cleanup: Split out irt_interfaces.cc from irt_ppapi.cc
> >
> > The intention here is that irt_interfaces.{cc,h} should list the NaCl
> > IRT interfaces that we implement in Chromium but that other files
> > (like irt_ppapi.cc) should implement them.  This matches how the files
> > are organised in native_client/src/untrusted/irt/.
> >
> > This is in preparation for copying the PNaCl translator IRT interfaces
> > to the Chromium side.  That will add a couple more interfaces to
> > irt_interfaces.cc (rather than to irt_ppapi.cc).
> >
> > BUG=302078
> > TEST=NaCl tests in browser_tests
> >
> > Committed: https://crrev.com/5e662010c1f7881970de6de19bcbd50f2726ea00
> > Cr-Commit-Position: refs/heads/master@{#339508}
>
> TBR=jvoung@chromium.org,bbudge@chromium.org,mseaborn@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=302078
>
> Committed: https://crrev.com/db9f0a6b277a39d2f2b70d99e17801a18a6d79a6
> Cr-Commit-Position: refs/heads/master@{#339559}

BUG=302078
TBR=jvoung@chromium.org

Committed: https://crrev.com/770b328e9f6812f4695e92963e0246ffd4f701a4
Cr-Commit-Position: refs/heads/master@{#339666}

[Mark Seaborn, 2015-07-21 04:51:21]

Created Reland of NaCl cleanup: Split out irt_interfaces.cc from irt_ppapi.cc

[Mark Seaborn, 2015-07-21 04:51:57]

mseaborn@chromium.org changed reviewers:
- bbudge@chromium.org

[Mark Seaborn, 2015-07-21 14:55:30]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2015-07-21 14:55:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1245893002/1

[jvoung (off chromium), 2015-07-21 16:36:07]

rubberstamp LGTM -- thanks for investigating

[commit-bot: I haz the power, 2015-07-21 16:38:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-21 16:39:47]

Patchset 1 (id:??) landed as
https://crrev.com/770b328e9f6812f4695e92963e0246ffd4f701a4
Cr-Commit-Position: refs/heads/master@{#339666}