Init Mojo embedder for Windows NaCl plugins.

Init Mojo embedder for Windows NaCl plugins.

With this change (and another fix for NACL_BUILD_SUBARCH), we can test
Pepper/Mojo/NaCl support on Windows.

This also moves embedder initialization to nacl_helper for clarity:
The Mojo embedder must be initialized in nacl_helper when that process
is used. When the plugin is running in a chrome process, ContentMain()
handles Mojo embedder initialization.

This change doesn't enable the test MojoFailsWithoutFlag on Windows,
since the abort() triggers a dialog that causes the test to time out.

BUG=414804
R=dmichael@chromium.org, ncbray@chromium.org

Committed: https://crrev.com/e30c97ba8d11ad3d6048dcc6167cbfcb7cf9d49f
Cr-Commit-Position: refs/heads/master@{#323908}

[teravest, 2015-04-03 19:52:40]

Resending this patch with MojoFailsWithoutFlag disabled on Windows.

The abort() in the disabled Mojo interface causes a modal dialog
"Abort/Retry/etc" to be raised, which causes the test to timeout. We can revisit
later if abort() is the right behavior when Mojo is disabled, but I think this
change is otherwise good.

Not sure why the CQ didn't catch that, but buildbots did...

[dmichael (off chromium), 2015-04-03 19:55:32]

FYI It's usually a good idea to either reuse the original CL or at least upload
the original as the first patchset to make it easier to see what changed.

lgtm

[Nick Bray (chromium), 2015-04-03 22:56:48]

LGTM

[teravest, 2015-04-06 15:22:21]

The CQ bit was checked by teravest@chromium.org

[commit-bot: I haz the power, 2015-04-06 15:22:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1059233004/1

[commit-bot: I haz the power, 2015-04-06 16:11:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-06 17:36:17]

Patchset 1 (id:??) landed as
https://crrev.com/e30c97ba8d11ad3d6048dcc6167cbfcb7cf9d49f
Cr-Commit-Position: refs/heads/master@{#323908}