heap: Trace until size for inlined storages on main thread

The recent change that got rid of clearing HeapVector's inlined storage
introduced an issue for the barrier, which may still invoke the regular
tracing routines for scanning on-stack inlined storages, which may run
into unitialized storage.

This CL distinguishes now main thread tracing from concurrent tracing.
For the main thread tracing, inlined storages are scanned until the
size to avoid tracing uninitilized on-stack slots.

Bug: 417904220
Change-Id: I617d01f5405bc8716dac99d88ce752667245cd1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6555883
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461282}