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Async call stack with async function always contains (async function) label and async function as top frame for following stack. We can merge label with this frame for better user experience.

R=dgozman@chromium.org,littledan@chromium.org
BUG=654018,621515

Review-Url: https://chromiumcodereview.appspot.com/2384533002
Cr-Commit-Position: refs/heads/master@{#426960}