Do not create the NativeViewport PlatformEventSource when headless.

Do not create the NativeViewport PlatformEventSource when headless.

The Chromium Linux Mojo bot's app_tests step started failing here:
  http://build.chromium.org/p/chromium.mojo/builders/Chromium%20Mojo%20Linux/builds/7090
Related to https://codereview.chromium.org/1130353008/

The mojo:view_manager_apptests all crash after passing with:
  [0518/132408:FATAL:x11_event_source.cc(80)] Check failed: display_.

These tests are run headless and shouldn't need an event source.

BUG=NONE
TEST=Bot goes green.
R=jam@chromium.org

Committed: https://crrev.com/67582621d7bfcb763c07d7a7605ba3bc200b4b5c
Cr-Commit-Position: refs/heads/master@{#330433}

[msw, 2015-05-18 21:21:55]

Hey John, please take a look; thanks!

[msw, 2015-05-18 21:22:04]

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

[jam, 2015-05-18 21:22:48]

lgtm

[commit-bot: I haz the power, 2015-05-18 21:23:14]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1125423005/1

[commit-bot: I haz the power, 2015-05-18 21:35:34]

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

[commit-bot: I haz the power, 2015-05-18 21:35:35]

Dry run: This issue passed the CQ dry run.

[msw, 2015-05-18 21:39:33]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-05-18 21:40:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1125423005/1

[commit-bot: I haz the power, 2015-05-18 21:46:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 22:44:15]

Patchset 1 (id:??) landed as
https://crrev.com/67582621d7bfcb763c07d7a7605ba3bc200b4b5c
Cr-Commit-Position: refs/heads/master@{#330433}