Bytecode flushing can make tests using assertOptimized flaky if the bytecode is
flushed between marking and optimization. It can also be flaky if the feedback vector
is collected before optimization. To prevent this, a new %PrepareForOptimization
runtime-test function is added that hold onto the bytecode strongly until it is
optimized after being explicitly marked for optimization by %OptimizeFunctionOnNextCall.

BUG=v8:8801,v8:8395

Change-Id: Idbd962a3a2044b915903f9c5e92d1789942b5b41
Reviewed-on: https://chromium-review.googlesource.com/c/1463525
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59914}