[Cast] Fix angle calculation in LinearChirp

Bug: None
Test: run cast_media_audio_unittests --gtest_filter=*PostProcessorTest.*
Change-Id: I742ed1cad68d81677d90b98682f1d473c05e20a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591915Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Mingyu Kang <kenkangxgwe@google.com>
Cr-Commit-Position: refs/heads/master@{#838750}

chromecast/media/cma/backend/mixer/post_processors/post_processor_unittest.cc

@@ -46,9 +46,9 @@ AlignedBuffer<float> LinearChirp(int frames,
   for (size_t ch = 0; ch < start_frequencies.size(); ++ch) {
     double angle = 0.0;
     for (int f = 0; f < frames; ++f) {
-      angle +=
-          start_frequencies[ch] +
-          (end_frequencies[ch] - start_frequencies[ch]) * f * M_PI / frames;
+      angle += (start_frequencies[ch] +
+                (end_frequencies[ch] - start_frequencies[ch]) * f / frames) *
+               M_PI;
       chirp[ch + f * start_frequencies.size()] = sin(angle);
     }
   }