Fix ewt_manifest.gyp to support both target and host toolchains

Fix ewt_manifest.gyp to support both target and host toolchains

Since base builds for both host and target so much ewt_manifest.
Without this gyp will fail when GYP_CROSSCOMPILE=1 and target_arch=arm:
gyp: Dependency 'S:\src\base\trace_event\etw_manifest\etw_manifest.gyp:etw_manifest#host' not found while trying to load target S:\src\base\base.gyp:base#host

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_nacl_sdk;tryserver.chromium.mac:mac_nacl_sdk
BUG=none

Committed: https://crrev.com/7c64f265edca18adefe01d0a4e676a3f7b834bd1
Cr-Commit-Position: refs/heads/master@{#327379}

[Sam Clegg, 2015-04-25 00:54:25]

sbc@chromium.org changed reviewers:
+ georgesak@chromium.org

[Sam Clegg, 2015-04-25 00:54:53]

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

[commit-bot: I haz the power, 2015-04-25 00:55:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1103943002/1

[commit-bot: I haz the power, 2015-04-25 04:13:24]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-25 04:13:25]

Dry run: Try jobs failed on following builders:
  win_nacl_sdk on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_nacl_sdk/buil...)

[Georges Khalil, 2015-04-28 17:30:22]

georgesak@chromium.org changed reviewers:
+ thestig@chromium.org

[Georges Khalil, 2015-04-28 17:30:23]

+thestig as reviewer.

[Lei Zhang, 2015-04-28 18:11:40]

lgtm

[Georges Khalil, 2015-04-28 18:22:59]

georgesak@chromium.org changed reviewers:
- georgesak@chromium.org

[Sam Clegg, 2015-04-28 18:35:28]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-04-28 18:35:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1103943002/1

[commit-bot: I haz the power, 2015-04-28 20:55:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-28 20:55:01]

Try jobs failed on following builders:
  win_nacl_sdk on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_nacl_sdk/buil...)

[Sam Clegg, 2015-04-28 21:09:49]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-04-28 21:11:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1103943002/1

[commit-bot: I haz the power, 2015-04-28 22:38:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 22:39:56]

Patchset 1 (id:??) landed as
https://crrev.com/7c64f265edca18adefe01d0a4e676a3f7b834bd1
Cr-Commit-Position: refs/heads/master@{#327379}