ConvolverNode buffer can be set multiple times

It is valid to set the buffer attribute more than once.

As resolved in https://github.com/WebAudio/web-audio-api/issues/1762,
setting the buffer more than once is allowed.

This is a revert (mostly) of
https://chromium-review.googlesource.com/c/chromium/src/+/1077713

Bug: 882211
Test: the-convolvernode-interface/convolver-setBuffer-already-has-value.html
Change-Id: Ie7f170883e6a6b2bfa7f14d89c74b72eebde3dac
Reviewed-on: https://chromium-review.googlesource.com/1249275
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595586}
3 files changed