Auto merge of #13305 - emilio:webgl-idl, r=nox,KiChjang

WebGL IDL and correctness.

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This is a change set with multiple intentions:
 * Be correct.
 * Unlock tests that are blocking @anholt.
 * Ease the transition to typed arrays once the changes by @Ms2ger start rolling
   in, since I expect the amount of test expectations to update to be
   non-trivial.

I expect having it to run on try a few times, but... r? @nox or @KiChjang

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