Consolidate echo canceller constants, if possible.

enum class EchoCancellationMode in web_media_stream_source.h
enum class EchoCancellationMode in media_stream_source.h
enum class EchoCancellationType in media_stream_audio_processor_options.h

And the strings:
kEchoCancellationType* in web_media_constraints.h
kEchoCancellationTypeValues in media_stream_constraints_util_audio_unittest.cc

See discussion in https://chromium-review.googlesource.com/c/chromium/src/+/1135005/7/third_party/blink/renderer/platform/mediastream/media_stream_source.h#75