Bug 1382545 - Part 1: Rounding the time of Animation API to 100ms when 'privacy.resistFingerprinting'is true. r=arthuredelstein,birtles

Adopt from Tor #16337.

This patch makes Animation API to report a rounded time when 'privacy.resistFingerprinting'
is true. The Animation API uses AnimationUtils::TimeDurationToDouble() to convert
its time duration into a double value and reports it when someone tries to query
time through Animation API. So, we use nsRFPService::ReduceTimePrecisionAsMSecs()
inside this method to round the time in the scope of the millisecond.

MozReview-Commit-ID: 8o01G6AlAu9

--HG--
extra : rebase_source : e2d52f04c1d63accb786f6cf9b1a102607eed517

dom/animation/AnimationUtils.h

@@ -10,6 +10,7 @@
 #include "mozilla/TimeStamp.h"
 #include "mozilla/dom/BindingDeclarations.h"
 #include "mozilla/dom/Nullable.h"
+#include "nsRFPService.h"
 #include "nsStringFwd.h"
 
 class nsIContent;
@@ -31,7 +32,9 @@ public:
     dom::Nullable<double> result;
 
     if (!aTime.IsNull()) {
-      result.SetValue(aTime.Value().ToMilliseconds());
+      result.SetValue(
+        nsRFPService::ReduceTimePrecisionAsMSecs(aTime.Value().ToMilliseconds())
+      );
     }
 
     return result;