Fix a numerical issue in NetEq delay plotting

Imprecisions in floating point representation caused noise in the
graphs. The integer division is in fact exact.

BUG= webrtc:7467

Review-Url: https://codereview.webrtc.org/2933053002
Cr-Commit-Position: refs/heads/master@{#18592}

webrtc/modules/audio_coding/neteq/tools/neteq_delay_analyzer.cc

@@ -14,6 +14,8 @@
 #include <limits>
 #include <utility>
 
+#include <webrtc/base/checks.h>
+
 namespace webrtc {
 namespace test {
 namespace {
@@ -115,8 +117,9 @@ void NetEqDelayAnalyzer::CreateGraphs(
   // This loop traverses data_ and populates rtp_timestamps_ms as well as
   // calculates the base offset.
   for (auto& d : data_) {
-    rtp_timestamps_ms.push_back(unwrapper.Unwrap(d.first) /
-                                (last_sample_rate_hz_ / 1000.f));
+    rtp_timestamps_ms.push_back(
+        unwrapper.Unwrap(d.first) /
+        rtc::CheckedDivExact(last_sample_rate_hz_, 1000));
     offset =
         std::min(offset, d.second.arrival_time_ms - rtp_timestamps_ms.back());
   }