Merge 177250 "Initialize value since calculateFinalValues may fa..."

Merge 177250 "Initialize value since calculateFinalValues may fa..."

> Initialize value since calculateFinalValues may fail to do so.
> 
> Fix threading issue where updateCoefficientsIfNecessary was not always
> called from the audio thread. This causes the value not to be
> initialized.
> 
> Thus,
> 
> o Initialize the variable to some value, just in case.
> o Split updateCoefficientsIfNecessary into two functions with the code
>   that sets the coefficients pulled out in to the new function
>   updateCoefficients.
> o Simplify updateCoefficientsIfNecessary since useSmoothing was always
>   true, and forceUpdate is not longer needed.
> o Add process lock to prevent the audio thread from updating the
>   coefficients while they are being read in the main thread. The audio
>   thread will update them the next time around.
> o Make getFrequencyResponse set the lock while reading the
>   coefficients of the biquad in preparation for computing the
>   frequency response.
> 
> BUG=389219
> 
> Review URL: https://codereview.chromium.org/354213002

TBR=rtoy@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178109

[Raymond Toy, 2014-07-14 23:14:59]

Committed patchset #1 manually as r178109 (tree was closed).