The ES2017 specification contains a so-called "throwaway" promise that
is used to specify the behavior of await in terms of PerformPromiseThen,
but it's actually not necessary and never exposed to user code. In
addition to that, hooking up the promise in await required a context (to
refer to the generator object) and two closures for the reject/fulfill
handling, which would resume the generator corresponding to the async
function. That meant, we had to allocate 4 additional objects for every
await.

Instead of using a JSPromise plus the callbacks, this CL adds logic to
allow PromiseReaction and PromiseReactionJobTask to carry arbitrary
payloads and Code handlers. We use this for await to avoid the
additional 4 objects mentioned above, and instead just have simple Code
handlers that resume the generator (for the async function), either by
throwing (in case of a rejection) or by resuming normally (in case of
fulfillment).

For this to work properly the JSGeneratorObject has to have a link to
the outer promise returned by the async function, so that the catch
prediction can still figure out what to do in case of promise rejection.
This is done by adding a new generator_outer_promise_symbol when the
debugger is active, which refers from the generator to the outer
promise.

With this change the doxbee-async-es2017-native test goes from around
100.54ms to around 82.45ms, which corresponds to a ~18% reduction in
execution time.

Bug: v8:7253
Change-Id: Iae25b3300bac351c3417be5ae687eff469b0e61f
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/924069Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51334}