Bug 1425462 Address tests for Time Jittering. r=bkelly

The following tests all hardcoded a special value for Timer Precision Reduction.

browser/components/extensions/test/xpcshell/test_ext_browsingData_cookies_cache.js
browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js
browser/components/resistfingerprinting/test/mochitest/test_animation_api.html
browser/components/resistfingerprinting/test/mochitest/test_reduce_time_precision.html
devtools/client/sourceeditor/test/browser_codemirror.js
dom/animation/test/css-animations/test_animation-currenttime.html
dom/animation/test/mozilla/test_transition_finish_on_compositor.html
dom/media/test/test_video_stats_resistfingerprinting.html
dom/tests/mochitest/ajax/jquery/test_jQuery.html
netwerk/test/unit/test_race_cache_with_network.js

Of these, only test_video_stats_resistfingerprinting.html begins failing when Jitter is enabled.
So disable jitter for that test.

Additionally, dom/midi/tests/test_midi_packet_timing_sorting.html began failing
with Jitter, so we disable it there. (We could easily modify the test so it began
passing, but this would reduce the effectiveness of the test.)

MozReview-Commit-ID: 2kipxV6wYv9

--HG--
extra : rebase_source : f455af2db1bba4e1c3986c413643b549ad29c208

dom/media/test/test_video_stats_resistfingerprinting.html

@@ -27,6 +27,7 @@ https://trac.torproject.org/projects/tor/ticket/15757
   gTestPrefs.push(
     ["privacy.resistFingerprinting", true],
     ["privacy.resistFingerprinting.reduceTimerPrecision.microseconds", 100000],
+    ["privacy.resistFingerprinting.reduceTimerPrecision.jitter", false],
     // We use 240p as the target resoultion since 480p is greater than every video
     // source in our test suite, so we need to use 240p here for allowing us to
     // test dropped rate here.

dom/midi/tests/test_midi_packet_timing_sorting.html

@@ -13,6 +13,7 @@
 
      async function runTests() {
        await MIDITestUtils.permissionSetup(true);
+       await SpecialPowers.pushPrefEnv({"set": [["privacy.resistFingerprinting.reduceTimerPrecision.jitter", false]]});
        var checkCount = 0;
        var lastTime = 0;
        var reopened = false;