b=890528 add more documentation and make length optional when createExpectedBuffers is present r=ehsan a=webaudio

--HG--
extra : rebase_source : cea34c4c9f9d9ca53855e8c262b220c4938487b6
test for bug 890528 a=webaudio

--HG--
extra : rebase_source : 3de58188880b1e3af58e6a324f7a4fcbd207339e
b=890528 delay buffer must be greater than max delay frames to avoid reading what has just been written r=ehsan a=webaudio

--HG--
extra : rebase_source : b31344eb8d26087fefdda4ff0fb619c647b30880
b=890528 Record silence in the delay buffer when there is no input, to avoid echos r=ehsan a=webaudio

--HG--
extra : rebase_source : 0a0db4ec6a7418d6d3328529248acd3fc238dd63
remove unintentional ! in ed23ecc80370 to test expected buffer frame count when length is supplied b=890528 a=webaudio

--HG--
extra : rebase_source : a5de2bb0ca2c07ee97f84f5ce3e03964eaed8bdd
use existing named constants b=890528 a=webaudio

--HG--
extra : rebase_source : 55ccc171e0a5d61df65e9c67a26e02127965f3e0