Add ASYNC_STEP support to trace_event and change START to ASYNC_BEGIN.

chrome/browser/resources/tracing/tests/start_finish.json

Index: chrome/browser/resources/tracing/tests/start_finish.json
diff --git a/chrome/browser/resources/tracing/tests/start_finish.json b/chrome/browser/resources/tracing/tests/start_finish.json
deleted file mode 100644
index c9053ba71cee68719ff123de3ea5a485e93e8813..0000000000000000000000000000000000000000
--- a/chrome/browser/resources/tracing/tests/start_finish.json
+++ /dev/null
@@ -1,22 +0,0 @@
-[
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