Shorten length of test for audiobuffersource-playbackrate.

Instead of generating almost 4 sec of audio, generate 1.8 sec instead. This makes the debug builds run this test much faster.

This is done by reducing the number of notes from 72 to 60 and making each note half as long as before. This shouldn't effect the accuracy of the test, except we're now missing the highest octave.

Also, testing on all platforms indicates that we only need just the one expected wav file, so the platform-specific versions have been removed.

BUG=349518
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/184343011

git-svn-id: svn://svn.chromium.org/blink/trunk@168852 bbb929c8-8fbe-4397-9dbb-9b2b20218538