When the --initial-heap-size flag is set V8's heap will grow to that
value without full GCs. Additionally, full GCs will be skipped
if the heap size drops below that value.

This patch also adds a comment for the initial_heap_size_in_bytes
parameter of ResourceConstraints:ConfigureDefaultsFromHeapSize

Change-Id: I85fda14bc6422af7bf2193d530efbc9b0bd0553d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1728622
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63054}