Update WebAudio layout tests to use unprefixed AudioContext.

All of the layout tests are updated to use unprefixed AudioContext and
OfflineAudioContext. Conversion was done using a script. All tests pass.

However, we still want to be able to run these tests on older browsers,
so where possible, we add a compatibility layer (in resources/compatibility.js) to
support running selected tests in browsers with prefixed AudioContext. This was
tested manually using Chrome 34.0.1847.60 beta on Linux.

BUG=

Review URL: https://codereview.chromium.org/208943004

git-svn-id: svn://svn.chromium.org/blink/trunk@169966 bbb929c8-8fbe-4397-9dbb-9b2b20218538