Replace assert() with RTC_DCHECK().

CL partially auto-generated with:

git grep -l "\bassert(" | grep "\.[c|h]" | \
  xargs sed -i 's/\bassert(/RTC_DCHECK(/g'

And with:

git grep -l "RTC_DCHECK(false)" |  \
  xargs sed -i 's/RTC_DCHECK(false)/RTC_NOTREACHED()/g'

With some manual changes to include "rtc_base/checks.h" where
needed.

A follow-up CL will remove assert() from Obj-C code as well
and remove the #include of <assert.h>.

The choice to replace with RTC_DCHECK is because assert()
is because RTC_DCHECK has similar behavior as assert()
based on NDEBUG.

This CL also contains manual changes to switch from
basic RTC_DCHECK to other (preferred) versions like
RTC_DCHECK_GT (and similar).

Bug: webrtc:6779
Change-Id: I00bed8886e03d685a2f42324e34aef2c9b7a63b0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/224846
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34442}

modules/audio_coding/neteq/sync_buffer.cc

@@ -28,7 +28,7 @@ void SyncBuffer::PushBack(const AudioMultiVector& append_this) {
     next_index_ -= samples_added;
   } else {
     // This means that we are pushing out future data that was never used.
-    //    assert(false);
+    //    RTC_NOTREACHED();
     // TODO(hlundin): This assert must be disabled to support 60 ms frames.
     // This should not happen even for 60 ms frames, but it does. Investigate
     // why.