Prevent ProcessProxy from passing bad fds to ProcessOutputWatcher

Prevent ProcessProxy from passing bad fds to ProcessOutputWatcher

This makes crosh process creation (used by terminalPrivate API) fail
gracefully when file descriptors used to observe process output
cannot be used by select (as opposed to crashing).

BUG=480616
TEST=Open huge number of crosh terminals in short time, until terminal
launching starts failing.

Committed: https://crrev.com/296875c4f5717f003005ccfd1e2ed0fd5675bb4b
Cr-Commit-Position: refs/heads/master@{#329901}

[tbarzic, 2015-05-14 19:01:25]

tbarzic@chromium.org changed reviewers:
+ xiyuan@chromium.org

[tbarzic, 2015-05-14 19:01:25]

Can you take a quick look at this?

[xiyuan, 2015-05-14 19:10:30]

lgtm

[tbarzic, 2015-05-14 19:18:09]

The CQ bit was checked by tbarzic@chromium.org

[commit-bot: I haz the power, 2015-05-14 19:21:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1135823007/20001

[commit-bot: I haz the power, 2015-05-14 19:40:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-14 19:42:17]

Patchset 2 (id:??) landed as
https://crrev.com/296875c4f5717f003005ccfd1e2ed0fd5675bb4b
Cr-Commit-Position: refs/heads/master@{#329901}