De-flake two animated custom property WPTs

These two tests use a duration of 1000s and a delay of -500s
to effectively start the animation at 50%. That's fine, but we're
not necessarily guaranteed that the animation hasn't progressed
at the time we're making the test assertions. Therefore, make
the animations initially "paused".

Making an animation initially paused can sometimes void a test,
if relevant behavior can only be exercised by a *playing* animation.
However, that is not an issue for these tests, since they primarily
care about cascade interaction which isn't affected by playing/paused.

Fixed: 385790290, 385816384
Change-Id: I409101af17670316aba7b0faa9104fc14dd3e755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6119955
Reviewed-by: Steinar H Gunderson <[email protected]>
Commit-Queue: Anders Hartvoll Ruud <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1401736}

css/css-cascade/revert-val-006.html

@@ -11,7 +11,7 @@
   }
   #h1 {
     margin-top: 0px;
-    animation: test linear 1000s -500s;
+    animation: test linear 1000s -500s paused;
   }
 </style>
 <h1 id=h1></h1>

css/css-properties-values-api/animation/custom-property-animation-used-in-shorthand.html

@@ -17,7 +17,7 @@
 }
 
 #target {
-  animation: angle-animation 1000s linear -500s;
+  animation: angle-animation 1000s linear -500s paused;
   background: linear-gradient(var(--angle), black, white);
 }