The GDB-stub for Wasm debugging (which builds with the flag
v8_enable_wasm_gdb_remote_debugging) doesn't build anymore after a few changes
in the interface of wasm::DebugInfo.
This CL fixes the build, and also adds a few small changes to the protocol.

Change-Id: I250a8c86fd83048434e68cbdc5cb8ae243577393
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2571341Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71669}

This changelist makes the GDB-stub actually execute GDB-remote commands, by
accessing the Wasm engine state. More precisely:
- class GdbServer registers DebugDelegates that receive debug notifications when
  a new Wasm module is loaded, when execution suspends at a breakpoint or for an
  unhandled exception.
- Since the GDB-remote commands arrive on a separate thread, all
  queries from the debugger are transformed into Task objects, that are posted
  into a TaskRunner that runs in the Isolate thread.
- class WasmModuleDebug contains the logic to retrieve the value of globals, locals, memory ranges from the
  Wasm engine and to add/remove breakpoints.

Build with: v8_enable_wasm_gdb_remote_debugging = true
Run with: --wasm-gdb-remote
Test with: python tools\run-tests.py --outdir=out\debug_x64 debugging -j 1

Bug: chromium:1010467
Change-Id: I9703894620a027d3c920926db92e2ff809d84ab8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1941139Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#67412}