This patch also cleans up NewPromiseCapability.

This patch results in a 20% improvement over 4 runs with the following micro
benchmark -

var x = Promise.resolve();

async function bar() {
    return x;
}

async function foo() {
    await bar();
}

var start = performance.now();
var count = 0;
var max = 10000;
for(var i = 0; i <= max; i++) {
    foo().then(() => {
        count++;
        if(count === max) print( performance.now() - start );
    })
}

BUG=v8:5639

Review-Url: https://codereview.chromium.org/2512103002
Cr-Commit-Position: refs/heads/master@{#41116}