Merge the list of concurrently swept ArrayBufferExtensions sooner back
to the main thread. When appending a new ArrayBufferExtension check
whether the concurrent sweeping was already finished and merge the lists
if it is.

In order to reduce the number of GCs in the linked test case, reset
young_bytes_ and old_bytes_ to 0 while sweeping the
ArrayBufferExtensions. Surviving extensions will be accounted again
when merging lists.

As a drive-by change remove scavenge.process_array_buffers from
GCTracer. GCTracer also printed the wrong value for fast_promote.

Bug: v8:11044
Change-Id: I8a772df895c43a69493015f42336c6f33fe52056
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2505764Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70880}