AEC3: Ensure that the high-pass filter is on whenever the AEC is active

This CL ensures that the high-pass filter is on whenever the echo
controller is on. This is important as the echo controller code assumes
that the external high-pass filter is active.

The CL also corrects the ToggleAec unit test (which started failing
after this code change).

Bug: webrtc:11159, chromium:1030179
Change-Id: Ief86eda8f7c67df1c25ac1a06d2cc0778e01196d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161228
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29998}

modules/audio_processing/include/audio_processing.h

@@ -283,6 +283,8 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
       // Recommended not to use. Will be removed in the future.
       bool use_legacy_aec = false;
       bool export_linear_aec_output = false;
+      // Enforce the highpass filter to be on (has no effect for the mobile
+      // mode).
       bool enforce_high_pass_filtering = true;
     } echo_canceller;