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.. as well as its header and unit test.

This is a prepration CL to Onion Soup the remaining bits
in //content/renderer/media_capture_from_element/ (namely
class HtmlAudioElementCapturerSource).

Note that this opens room for some possible follow ups, including

- the rename of the files itself, using the convension in Blink,
  eg web_audio_source_provider_impl.cc|h|test.cc

- considering unifying webaudiosourceprovider_impl.cc|h and
  web_audio_source_provider.h now that they are sibling headers,
  or alternatively rename it to something that indicates it also
  implements media::SwitchableAudioRendererSink.

BUG=787261
R=dalecurtis@chromium.org, guidou@chromium.org, haraken@chromium.org

Change-Id: Ifd1da4e7001f39c1740e2df94d723cbca26f160d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629948Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#665033}