Always properly release CaptureEventProducer::shutdown_requested_mutex_
Always properly release CaptureEventProducer::shutdown_requested_mutex_

The mutex wasn't released when acquired after the timeout (that is, when
`shutdown_requested_` had *NOT* become `true`).
This is now in line with the same code around thirty lines below.

Bug: http://b/197310492

Test: Run unit tests. Capture OrbitTest with manual instrumentation (which uses
`CaptureEventProducer`).

google/orbit,google/orbit,google/orbit,google/orbit