stream: rename poorly named function

roundUpToNextPowerOf2() does more than just rounding up to the next
power of two.  Rename it to computeNewHighWaterMark().

PR-URL: nodejs#2479
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

lib/_stream_readable.js

@@ -192,7 +192,7 @@ Readable.prototype.setEncoding = function(enc) {
 
 // Don't raise the hwm > 8MB
 const MAX_HWM = 0x800000;
-function roundUpToNextPowerOf2(n) {
+function computeNewHighWaterMark(n) {
   if (n >= MAX_HWM) {
     n = MAX_HWM;
   } else {
@@ -231,7 +231,7 @@ function howMuchToRead(n, state) {
   // power of 2, to prevent increasing it excessively in tiny
   // amounts.
   if (n > state.highWaterMark)
-    state.highWaterMark = roundUpToNextPowerOf2(n);
+    state.highWaterMark = computeNewHighWaterMark(n);
 
   // don't have that much.  return null, unless we've ended.
   if (n > state.length) {