This CL adds a new method intended for tests or lab settings to
cleanup V8 caches. The synchronous nature of the method greatly reduces
flakiness of blink leak detection in many cases.

Bug: chromium:1217831
Change-Id: I107eddc8b88d91aa7e69430ecfc135fe39538a5c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2948666
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75041}