[NaCl SDK] Don't exit from nacl_io_test before posting message to JS.

[NaCl SDK] Don't exit from nacl_io_test before posting message to JS.

In the nacl_io_test there's a race condition between the NaCl module exiting
and a posted message from the module being handled by JavaScript.

To fix it, when running in the browser, we wait indefinitely and assume the
JavaScript will kill the module when it is ready.

(This fixes a flake that can be seen in
http://build.chromium.org/p/client.nacl.sdk/builders/mac-sdk-multi/builds/5838/steps/Run%20Tests/logs/stdio

BUG=none
R=sbc@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=222065

[Sam Clegg, 2013-09-04 22:31:09]

lgtm, although we should probably have a preferred way
for ppapi_simple apps to exit cleanly including the
flushing of all messages.

Perhaps ppapi_simple should not actually call exit() at
all but instead just let the application thread end and
leave the main thread running forever?

[binji, 2013-09-06 20:41:39]

OK, I'll land as-is. We should revisit how to handle this in ppapi_simple later.

[binji, 2013-09-09 18:50:59]

Committed patchset #1 manually as r222065 (presubmit successful).