Don't reference the files in Source/platform/audio/resources from the
tests.  Copy them to webaudio/resources/hrtf.  And add a README to
indicate where these files came from.  The README in
platform/audio/resources already references this directory, so we're
all set.

Also removed the files from BUILD.gn, reverting the previous
change.

BUG=693838,
TEST=hrtf-database.html

Review-Url: https://codereview.chromium.org/2741543002
Cr-Commit-Position: refs/heads/master@{#455853}