Clean up custom element default style perf tests

These tests were in a group that was timing out on android servers.
I converted both of them from PerfTestRunner.measureTime to
PerfTestRunner.measureRunsPerSecond(), which should be more predictable
across platforms. I also cleaned up the code a bit.

* blink_perf.dom/custom-element-default-style-with-shadow.html
* blink_perf.dom/custom-element-default-style.html

Metrics for these two tests *will* change after this CL lands,
but that should not be a concern.

Bug: 1142315
Change-Id: I80bc469553621420eb2e3f8b250d96260e8515b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505744
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822062}

third_party/blink/perf_tests/dom/custom-element-default-style-with-shadow.html

@@ -12,23 +12,17 @@ customElements.define('my-element', class MyElement extends HTMLElement {
   }
 });
 
-PerfTestRunner.measureTime({
+PerfTestRunner.measureRunsPerSecond({
     description: "Measures performance of styling custom elements with shadowRoot's innerHTML.",
 
-    setup: function() {
-      while (holderElement.innerHTML = '') {
-        holderElement.removeChild(holderElement.firstChild);
-      }
-    },
-
     run: function() {
+      holderElement.textContent = '';
       for (i = 0; i < 100; i++) {
         holderElement.appendChild(document.createElement('my-element'));
         // force layout.
         document.body.offsetWidth;
       }
     },
-    warmUpCount: 3,
-    iterationCount: 500,
+    warmUpCount: 1,
 });
 </script>

third_party/blink/perf_tests/dom/custom-element-default-style.html

@@ -13,23 +13,18 @@ window.customElements.define("element-constructed", class extends HTMLElement {
   }
 }, { style: constructedStyleSheet });
 
-PerfTestRunner.measureTime({
+PerfTestRunner.measureRunsPerSecond({
     description: "Measures performance of styling custom elements with default style.",
 
-    setup: function() {
-      while (holderElement.innerHTML = '') {
-        holderElement.removeChild(holderElement.firstChild);
-      }
-    },
-
     run: function() {
+      holderElement.textContent = '';
       for (i = 0; i < 100; i++) {
         holderElement.appendChild(document.createElement('element-constructed'));
         // force layout.
         document.body.offsetWidth;
       }
     },
-    warmUpCount: 3,
-    iterationCount: 500,
+
+    warmUpCount: 1,
 });
 </script>