PNaCl: Hold subprocess_mu while doing StartSrpcServices.

PNaCl: Hold subprocess_mu while doing StartSrpcServices.

StartSrpcServices will require using the service_runtime pointer,
to set up the srpc_client pointer. So we cannot destroy the
service_runtime at the same time.

E.g., if I add a sleep() to induce the race, I get:

../../native_client/src/include/nacl_scoped_ptr.h:96: C *nacl::scoped_ptr<plugin::SelLdrLauncherChrome>::operator->() const [C = plugin::SelLdrLauncherChrome]: Assertion `ptr_ != __null' failed.
Received signal 6
#0 0x7f687f4aef7e base::debug::StackTrace::StackTrace()
#1 0x7f687f4aeabf base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f6879dee340 <unknown>
#3 0x7f6876504cc9 gsignal
#4 0x7f68765080d8 abort
#5 0x7f68764fdb86 <unknown>
#6 0x7f68764fdc32 __assert_fail
#7 0x7f688e2887f2 nacl::scoped_ptr<>::operator->()
#8 0x7f688e288487 plugin::ServiceRuntime::SetupAppChannel()
#9 0x7f688e28cb76 plugin::NaClSubprocess::StartSrpcServices()
#10 0x7f688e281810 plugin::PnaclTranslateThread::DoLink()
#11 0x7f688e27fe7d plugin::PnaclTranslateThread::DoLinkThread()
#12 0x7f6879de6182 start_thread
#13 0x7f68765c847d clone

Should have checked this in previous refactoring:
https://codereview.chromium.org/1128943003

BUG=473474
Committed: https://crrev.com/7862c2537ff491d5865a451b3f9f96bb7c2f3483
Cr-Commit-Position: refs/heads/master@{#329718}

[jvoung (off chromium), 2015-05-13 17:26:26]

jvoung@chromium.org changed reviewers:
+ dschuff@chromium.org

[Derek Schuff, 2015-05-13 20:56:31]

lgtm

[jvoung (off chromium), 2015-05-13 21:06:44]

The CQ bit was checked by jvoung@chromium.org

[commit-bot: I haz the power, 2015-05-13 21:08:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128373005/1

[commit-bot: I haz the power, 2015-05-13 21:16:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-13 21:17:05]

Patchset 1 (id:??) landed as
https://crrev.com/7862c2537ff491d5865a451b3f9f96bb7c2f3483
Cr-Commit-Position: refs/heads/master@{#329718}