Introduce dchecked_cast, and start using it

It's the faster, less strict cousin of checked_cast.

BUG=none

Review-Url: https://codereview.webrtc.org/2714063002
Cr-Commit-Position: refs/heads/master@{#16958}
diff --git a/webrtc/modules/audio_coding/acm2/acm_receiver.cc b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
index fd4e91a..1577d2d 100644
--- a/webrtc/modules/audio_coding/acm2/acm_receiver.cc
+++ b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
@@ -162,7 +162,7 @@
     audio_frame->sample_rate_hz_ = desired_freq_hz;
     RTC_DCHECK_EQ(
         audio_frame->sample_rate_hz_,
-        rtc::checked_cast<int>(audio_frame->samples_per_channel_ * 100));
+        rtc::dchecked_cast<int>(audio_frame->samples_per_channel_ * 100));
     resampled_last_output_frame_ = true;
   } else {
     resampled_last_output_frame_ = false;