Bug 1387894 - Disable unconditional clamping for two CSS tests r=birtles

We're hitting double imprecision here.

Example: Given 86.68392200000000 and 86.67999999999999 we want to see
if they're equal with two significant digits. (They are, they're 86.68)

However, when we reduce them, 86.68 (which is represented as an epislon lower)
gets reduced to 86.66 and they no longer match. We disable unconditional clamping
on these tests to confirm they behave the way they shoud... excepting the clamping
which may introduce imprecision.

Differential Revision: https://phabricator.services.mozilla.com/D38812

--HG--
extra : moz-landing-system : lando

testing/web-platform/meta/css/css-animations/CSSAnimation-startTime.tentative.html.ini

@@ -0,0 +1,2 @@
+[CSSAnimation-startTime.tentative.html]
+  prefs: [privacy.reduceTimerPrecision.unconditional:false]

testing/web-platform/meta/css/css-transitions/CSSTransition-startTime.tentative.html.ini

@@ -1,4 +1,5 @@
 [CSSTransition-startTime.tentative.html]
+  prefs: [privacy.reduceTimerPrecision.unconditional:false]
   disabled:
     if (os == "android") and debug: https://bugzilla.mozilla.org/show_bug.cgi?id=1560466