Forced GCs can either be invoked internally or communicate the fact that
they are forced externally via API. Before this CL, all uses were
passing kGCCallbackFlagForced to indicate that the GC was forced.

This flag is used by embedders though to trigger followup actions. E.g.,
it can be used to trigger a follow up call to
GarbageCollectionForTesting() call which requires --expose-gc.

This patch changes the semantics as follows:
- Internal forced GCs use a Heap GC flag (kForcedGC)
- External forced GCs and GC extension use kGCCallbackFlagForced

Bug: chromium:1074061
Change-Id: Ide7ea0ccdf88b8c8cac002289aef5b7eb0f9748c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2172747Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67498}