bug 1215096 correct off-by-one error in playback position of resampled buffers r=padenot

"The behavior of an expression of the form E1 op = E2 is equivalent to E1 = E1
op E2 except that E1 is evaluated only once", which means that the subtraction
of -= was happening before conversion from double to unsigned int.

The "+ 0.5" was subtracted before the truncation toward zero, causing rounding
to nearest minus one, except when nearest was zero.

--HG--
extra : rebase_source : 3b2335da7a244245ea2fcf5c80760dc1645e6dae

dom/media/webaudio/AudioBufferSourceNode.cpp

@@ -270,7 +270,8 @@ public:
         if (leadTicks > 0.0) {
           // Round to nearest output subsample supported by the resampler at
           // these rates.
-          skipFracNum -= leadTicks * ratioNum + 0.5;
+          uint32_t leadSubsamples = leadTicks * ratioNum + 0.5;
+          skipFracNum -= leadSubsamples;
           MOZ_ASSERT(skipFracNum < INT32_MAX, "mBeginProcessing is wrong?");
         }
         speex_resampler_set_skip_frac_num(resampler, skipFracNum);