To consume a code cache off-thread

  1. The embedder creates a CachedData object wrapping the data blob.
  2. The embedder calls ScriptCompiler::StartConsumingCodeCache with the
     CachedData, and receives a ScriptCompiler::CodeCacheConsumeTask
     which takes ownership of the CachedData.
  3. The embedder calls ScriptCompiler::CodeCacheConsumeTask::Run
     on a different thread.
  4. Once this completes, the embedded passes the completed task as an
     optional argument into Source constructor, and calls Compile as
     before.

This is roughly similar to how streaming compilation works, with the
QoL improvement that Source owns the CodeCacheConsumeTask and therefore
we can reuse the same Compile method and do the off-thread finalization
behind the scenes inside Compile.

On the v8::internal side, ScriptCompiler::CodeCacheConsumeTask wraps a
v8::internal::BackgroundDeserializeTask, which has a Run and a Finish
method. The Run creates a LocalIsolate (again, similar to
BackgroundCompileTask), calls some helpers on CodeSerializer, and stores
the pre-finalization result in a OffThreadDeserializeData structure.
This stores Persistent Handles to the off-thread initialized SFI and
a vector of Scripts needing fixing up, and it owns the PersistentHandles
object which owns those Handles. Finally, the Finish method consumes
this OffThreadDeserializeData structure, fixes up Scripts, moves the
SFI Handle into the caller HandleScope, and that's it.

Since we don't yet have the source at off-thread deserialization time,
the various code cache sanity checks are done without the source hash
when deserializing, and the Finish method re-does them now that the
source is available.

Bug: chromium:1075999
Change-Id: If1faf35ba3ef840fa4e735581d0b29c96c1d5fc8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3067322
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76155}