AcmReceiver: Make a member variable const

This is a minor clean-up made possible by simplifications done in the
past.

Bug: none
Change-Id: Id0ea167572f8da36db5de949441f67a2a18555be
Reviewed-on: https://chromium-review.googlesource.com/528073
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18490}

webrtc/modules/audio_coding/acm2/acm_receiver.cc

@@ -37,7 +37,7 @@ AcmReceiver::AcmReceiver(const AudioCodingModule::Config& config)
       neteq_(NetEq::Create(config.neteq_config, config.decoder_factory)),
       clock_(config.clock),
       resampled_last_output_frame_(true) {
-  assert(clock_);
+  RTC_DCHECK(clock_);
   memset(last_audio_buffer_.get(), 0, AudioFrame::kMaxDataSizeSamples);
 }