Bug 1440195 For timestamps that are absolute, specify a null context pointer r=baku

Note that by not using the same context pointer for all timestamps within a single
'communication group' (that is, all things that can communication to each other
in JavaScript), it's possible to observe time going backwards.

Imagine comparing
  performance.timeOrigin + performance.now() < new File([], "").lastModified

In theory this should always be true. However, if performance.now() was jittered
up (using one context pointer, because it is a relative timestamp) and File was
jittered down (using a null context pointer, because it is an absolute timestamp)
then this may evaluate to False.

I think this is okay.

MozReview-Commit-ID: BfgbmGS8XdD

--HG--
extra : rebase_source : 1d105d9b63e61561be06dbfe2a3fccf534ee68b3

dom/file/BaseBlobImpl.cpp

@@ -64,7 +64,8 @@ BaseBlobImpl::GetLastModified(ErrorResult& aRv)
 {
   MOZ_ASSERT(mIsFile, "Should only be called on files");
   if (IsDateUnknown()) {
-    mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(PR_Now());
+    mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(PR_Now(), 0);
+    // mLastModificationDate is an absolute timestamp so we supply a zero context mix-in
   }
 
   return mLastModificationDate / PR_USEC_PER_MSEC;

dom/file/MultipartBlobImpl.cpp

@@ -269,7 +269,8 @@ MultipartBlobImpl::SetLengthAndModifiedDate(ErrorResult& aRv)
     //   x.getTime() < f.dateModified.getTime()
     // could fail.
     mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(
-      lastModifiedSet ? lastModified * PR_USEC_PER_MSEC : JS_Now());
+      lastModifiedSet ? lastModified * PR_USEC_PER_MSEC : JS_Now(), 0);
+    // mLastModificationDate is an absolute timestamp so we supply a zero context mix-in
   }
 }
 

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -557,7 +557,7 @@ nsRFPService::ReduceTimePrecisionAsUSecsWrapper(double aTime)
     aTime,
     MicroSeconds,
     TimerResolution(),
-    0,
+    0, /* For absolute timestamps (all the JS engine does), supply zero context mixin */
     TimerPrecisionType::All);
 }