This adds the following trace events to help diagnose GC scheduling
issues:
- V8.MemoryPressureNotification
- V8.CheckMemoryPressure
- V8.ExternalMemoryPressure

Bug: chromium:1072746
Change-Id: If850fc3d0f419395f22ec05b1779797916e39798
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2159487Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67311}