Fix a shutdown crash under CFAutoreleasePoolPop

NSAutoreleasePool init and dray must be symetrical and we can't guarantee
that for ContentMainRunner as well as Chromium can since we initialize
the ContentMainRunner on-demand but destroy it with the QCoreApplication's
destruction.

We also don't need to use it since any allocation that
ContentMainRunnerImpl::autorelease_pool_ would cover is already covered
at the bottom of the stack by the QCocoaAutoReleasePool in
QCocoaEventDispatcher::processEvents.

Change-Id: I2874916420457e3d36d08fb9fca0d919f374f592
Reviewed-by: Andras Becsi <andras.becsi@digia.com>