Reland "Remove Dezippering from BiquadFilter"

This is a reland of 53b1b7da.

Original change's description:
> Remove Dezippering from BiquadFilter
> 
> Remove dezippering from all of the attributes.  The value will now 
> change immediately instead of gradually changing from the old value
> to the new value.
> 
> Chromium Feature: https://www.chromestatus.com/features/5287995770929152
> Intent to ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/YKYRrh0nWMo/aGzd3049AgAJ
> 
> Bug: 752986
> Test: BiquadFilter/dezipper.html
> Change-Id: I86840159709158fd52a69a7c3c5f279267277d44
> Reviewed-on: https://chromium-review.googlesource.com/612104
> Commit-Queue: Raymond Toy <rtoy@chromium.org>
> Reviewed-by: Hongchan Choi <hongchan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#532910}

Bug: 752986
Change-Id: Ia911c6e93d7dd7da1dea6290ac4201f81f42da3a
Reviewed-on: https://chromium-review.googlesource.com/894243Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533314}

third_party/WebKit/Source/modules/webaudio/BiquadDSPKernel.cpp

@@ -52,10 +52,10 @@ void BiquadDSPKernel::UpdateCoefficientsIfNecessary(int frames_to_process) {
           detune, frames_to_process);
       UpdateCoefficients(frames_to_process, cutoff_frequency, q, gain, detune);
     } else {
-      cutoff_frequency[0] = GetBiquadProcessor()->Parameter1().SmoothedValue();
-      q[0] = GetBiquadProcessor()->Parameter2().SmoothedValue();
-      gain[0] = GetBiquadProcessor()->Parameter3().SmoothedValue();
-      detune[0] = GetBiquadProcessor()->Parameter4().SmoothedValue();
+      cutoff_frequency[0] = GetBiquadProcessor()->Parameter1().Value();
+      q[0] = GetBiquadProcessor()->Parameter2().Value();
+      gain[0] = GetBiquadProcessor()->Parameter3().Value();
+      detune[0] = GetBiquadProcessor()->Parameter4().Value();
       UpdateCoefficients(1, cutoff_frequency, q, gain, detune);
     }
   }

third_party/WebKit/Source/modules/webaudio/BiquadProcessor.cpp

@@ -80,6 +80,16 @@ void BiquadProcessor::CheckForDirtyCoefficients() {
       filter_coefficients_dirty_ = true;
       has_just_reset_ = false;
     } else {
+      // TODO(crbug.com/763994): With dezippering removed, we don't want to use
+      // these methods.  We need to implement another way of noticing if one of
+      // the parameters has changed.  We do this as an optimization because
+      // computing the filter coefficients from these parameters is fairly
+      // expensive.  NB: The calls to Smooth() don't actually cause the
+      // coefficients to be dezippered.  This is just a way to notice that the
+      // coefficient values have changed.  |UpdateCoefficientsIfNecessary()|
+      // checks to see if the filter coefficients are dirty and sets the filter
+      // to the new value, without smoothing.
+      //
       // Smooth all of the filter parameters. If they haven't yet converged to
       // their target value then mark coefficients as dirty.
       bool is_stable1 = parameter1_->Smooth();

third_party/WebKit/Source/platform/audio/Biquad.cpp

@@ -840,7 +840,16 @@ double Biquad::TailFrame(int coef_index, double max_frame) {
       DCHECK(std::isfinite(c2));
 
       tail_frame = 1 + log(kMaxTailAmplitude / (c1 + c2)) / log(r);
-      DCHECK(std::isfinite(tail_frame));
+      if (c1 == 0 && c2 == 0) {
+        // If c1 = c2 = 0, then H(z) = b0.  Hence, there's no tail
+        // because this is just a wire from input to output.
+        tail_frame = 0;
+      } else {
+        // Otherwise, check that the tail has finite length.  Not
+        // strictly necessary, but we want to know if this ever
+        // happens.
+        DCHECK(std::isfinite(tail_frame));
+      }
     }
   } else {
     // Repeated roots.  This should be pretty rare because all the