Updated PlayAudio test in web_engine_integration_tests to enable
FuchsiaAudioConsumer and verify that the audio played as expected. It
uses a fake implementation of fuchsia.media.AudioConsumer interface
that's injected to the web context.
Also updated the tests to play two files with different sample rate
to verify that the audio renderer handles mid-stream config changes.

Bug: 1027048
Change-Id: I14b0b63d9dc7444c749ee0da348b39df93c1242b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940726Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756451}