[LayoutInstability] Fix failures in tests shifting in/out of viewport

Currently, some tests work under the assumption that the height of the
viewport is exactly 600px. While this is true for tests running on our
bots, it's not true for tests running on WPT.fyi. Thus, this assumption
is removed from those tests.

Bug: 1036013
Change-Id: I34b057730e22bc208cbb958d179195445def8b77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1984678Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728280}

third_party/blink/web_tests/external/wpt/layout-instability/shift-into-viewport.html

@@ -4,7 +4,7 @@
 <style>
 
 body { margin: 0; }
-#j { position: relative; width: 600px; height: 200px; top: 600px; }
+#j { position: absolute; width: 600px; height: 200px; top: 100%; }
 
 </style>
 <div id='j'></div>
@@ -19,7 +19,9 @@ promise_test(async () => {
   // Wait for the initial render to complete.
   await waitForAnimationFrames(2);
 
-  document.querySelector("#j").style.top = "400px";
+  // Move div partially into viewport.
+  document.querySelector("#j").style.top =
+      document.documentElement.clientHeight - 200 + "px";
 
   // The element moves from outside the viewport to within the viewport, which
   // should generate a shift.

third_party/blink/web_tests/external/wpt/layout-instability/shift-outside-viewport.html

@@ -4,7 +4,7 @@
 <style>
 
 body { margin: 0; }
-#j { position: relative; width: 600px; height: 200px; top: 600px; }
+#j { position: absolute; width: 600px; height: 200px; top: 100%; }
 
 </style>
 <div id='j'></div>
@@ -19,7 +19,9 @@ promise_test(async () => {
   // Wait for the initial render to complete.
   await waitForAnimationFrames(2);
 
-  document.querySelector("#j").style.top = "800px";
+  // Move div even further out of viewport.
+  document.querySelector("#j").style.top =
+      document.documentElement.clientHeight + 200 + "px";
 
   // Since the element moves entirely outside of the viewport, it shouldn't
   // generate a score.