Fix incorrect DCHECK in GetFrequencyResponse

The DCHECK was checking that the arrays have positive lengths, but the
spec allows zero-length arrays too.  So change the spec to check for
non-negative values.  And also modify getFrequencyResponse to return
quickly if the length is 0.

This applies to both BiquadFilterNodes and IIRFilterNodes.

Bug: 1073557
Change-Id: I8c98cf01021c9ece5731ccf8fdbc1fe8fe97f39c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163054Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762019}

third_party/blink/renderer/modules/webaudio/biquad_dsp_kernel.cc

@@ -195,7 +195,7 @@ void BiquadDSPKernel::GetFrequencyResponse(BiquadDSPKernel& kernel,
   // updating |kernel| while we're computing the response.
   DCHECK(IsMainThread());
 
-  DCHECK_GT(n_frequencies, 0);
+  DCHECK_GE(n_frequencies, 0);
   DCHECK(frequency_hz);
   DCHECK(mag_response);
   DCHECK(phase_response);

third_party/blink/renderer/modules/webaudio/biquad_filter_node.cc

@@ -284,9 +284,13 @@ void BiquadFilterNode::getFrequencyResponse(
         "frequencyHz length exceeds the maximum supported length");
     return;
   }
+
+  // If the length is 0, there's nothing to do.
+  if (frequency_hz_length_as_int > 0) {
     GetBiquadProcessor()->GetFrequencyResponse(
         frequency_hz_length_as_int, frequency_hz.View()->Data(),
         mag_response.View()->Data(), phase_response.View()->Data());
+  }
 }
 
 void BiquadFilterNode::ReportDidCreate() {

third_party/blink/renderer/modules/webaudio/iir_dsp_kernel.cc

@@ -28,7 +28,7 @@ void IIRDSPKernel::GetFrequencyResponse(int n_frequencies,
                                         const float* frequency_hz,
                                         float* mag_response,
                                         float* phase_response) {
-  DCHECK_GT(n_frequencies, 0);
+  DCHECK_GE(n_frequencies, 0);
   DCHECK(frequency_hz);
   DCHECK(mag_response);
   DCHECK(phase_response);

third_party/blink/renderer/modules/webaudio/iir_filter_node.cc

@@ -276,9 +276,13 @@ void IIRFilterNode::getFrequencyResponse(
         "frequencyHz length exceeds the maximum supported length");
     return;
   }
+
+  // Nothing to do if the length is 0.
+  if (frequency_hz_length_as_int > 0) {
     GetIIRFilterProcessor()->GetFrequencyResponse(
         frequency_hz_length_as_int, frequency_hz.View()->Data(),
         mag_response.View()->Data(), phase_response.View()->Data());
+  }
 }
 
 void IIRFilterNode::ReportDidCreate() {