[blink web_tests] Rework WebAssembly cache test

- Use async/await instead of promise chains.
- Properly start and stop Timeline, so events can propagate.
- Reenables test.

Bug: chromium:927296,chromium:719172
Change-Id: I33f676e1993ca07380bc645606b422b45f4b090d
Reviewed-on: https://chromium-review.googlesource.com/c/1450616
Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628999}
3 files changed