Shorten duration of audio in audiobuffersource-loop-points WebAudio test

This test generates 7+ sec of audio, which takes a long time on debug
builds.  Reduce the length to 1.8 sec by making the notes much shorter and only
generating 60 notes (5 octaves) instead of 72. This shouldn't affect
the quality of the test.

Running local tests for linux, mac, win, and android shows that all of the platform tests are identical and can be removed in favor of just the one reference.

BUG=312910,309197
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/191653002

git-svn-id: svn://svn.chromium.org/blink/trunk@168942 bbb929c8-8fbe-4397-9dbb-9b2b20218538