Use const and static_cast for priming FIFO in AudioDestination

A follow up CL from:
https://chromium-review.googlesource.com/c/chromium/src/+/1015201

Bug: 833766
Change-Id: Iaabea9e2801a43257871eeee2013e91fdb0f3d20
Reviewed-on: https://chromium-review.googlesource.com/1017003Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551759}

third_party/blink/renderer/platform/audio/audio_destination.cc

@@ -85,8 +85,9 @@ AudioDestination::AudioDestination(AudioIOCallback& callback,
 
   // Primes the FIFO for the given callback buffer size. This is to prevent
   // first FIFO pulls from causing "underflow" errors.
-  unsigned priming_render_quanta =
-      ceil(callback_buffer_size_/(float)AudioUtilities::kRenderQuantumFrames);
+  const unsigned priming_render_quanta =
+      ceil(callback_buffer_size_ /
+           static_cast<float>(AudioUtilities::kRenderQuantumFrames));
   for (unsigned i = 0; i < priming_render_quanta; ++i) {
     fifo_->Push(render_bus_.get());
   }