Bug 1424341 Add privacy.reduceTimerPrecision and privacy.reduceTimerPrecision.microseconds prefs r=bkelly,timhuang

This pref does not override privacy.resistFingerprinting, but when it is set (and
privacy.resistFingerprinting is not) we will still adjust the precision of almost
all timers. The adjustment amount is the second pref, which is defaulted to
20us but now dynamically adjustable (in the scale of microseconds.)

This patch does _not_ address the performance API, which privacy.resistFingerprinting
disables.

We are landing this preffed on at the current value we clamp performance.now() at
which is 20us.

MozReview-Commit-ID: ESZlSvH9w1D

modules/libpref/Preferences.cpp

@@ -5016,7 +5016,7 @@ Preferences::AddAtomicUintVarCache(Atomic<uint32_t, Order>* aCache,
 
 // Since the definition of template functions is not in a header file, we
 // need to explicitly specify the instantiations that are required. Currently
-// only the order=Relaxed variant is needed.
+// limited orders are needed and therefore implemented.
 template nsresult
 Preferences::AddAtomicBoolVarCache(Atomic<bool, Relaxed>*, const char*, bool);
 
@@ -5040,6 +5040,11 @@ Preferences::AddAtomicUintVarCache(Atomic<uint32_t, Relaxed>*,
                                    const char*,
                                    uint32_t);
 
+template nsresult
+Preferences::AddAtomicUintVarCache(Atomic<uint32_t, ReleaseAcquire>*,
+                                   const char*,
+                                   uint32_t);
+
 static void
 FloatVarChanged(const char* aPref, void* aClosure)
 {