Added validation of the input volume, if its too low fail the test.
Added testing of input channels to determine how many are active and display the volume.
Fixed according to comments, improved success messages, removed redundant input check.
added comment justifying the large buffer size
Merge pull request #194 from GoogleChrome/checkMicInputLevel

TestRTC: Improve mic test by adding fail criteria based on input volume and also active input channel detection.