Currently, debugger pauses on async call schedule and then waits for Debugger.pauseOnAsyncCall
with parentStackTraceId to actually schedule the pause.

This CL combines these two steps:
- For local async tasks, it just stores m_taskWithScheduledBreak at the time of schedule,
  to be able to pause once this task is run.
- For external async tasks, it plumbs "should_pause" boolean in V8StackTraceId from
  the point of schedule to the point of execution, and schedules a pause once
  externalAsyncTaskStarted is called with "should_pause" set to true.

This approach greatly simplifies the implementation, and reduced frontend to a single
"breakOnAsyncCall: true" parameter in Debugger.stepInto.

Drive-by: introduce hasScheduledBreakOnNextFunctionCall() to make
SetBreakOnNextFunctionCall management more robust.

Note: artificial pauses at async call schedule time are gone from test expectations -
we now only pause when user actually wants to pause, which makes protocol much simpler.

See also design doc linked in the bug.

BUG=chromium:1000475

Change-Id: I2d16f79c599fe196b2aaeca8223c63437a2954a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1783724
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63737}