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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}