Some cctests force fresh creation of heap constants, even though the
cctest binary itself is an embedded snapshot build (i.e.: a snapshot
blob exists, and a binary-embedded blob exists). This breaks a few
assumptions, for example that off-heap builtins have a single,
canonical off-heap code range.

Unfortunately this isn't that easy to fix. I see a few alternatives:

1. In builtins setup, if an embedded blob exists, regenerate the
builtins for their metadata (things like the safepoint table offset),
and then replace them by off-heap trampolines.

2. As above, but deserialize the trampolines from the snapshot blob.

3. As above, but pack required metadata into the embedded blob and
create trampolines from there.

4. Act as if the embedded blob does not exist.

Alternative 1 does not work because the generated code can be slightly
different at at runtime vs. mksnapshot-time. Alternative 2 is out
because we do not have access to the snapshot blob in TestIsolate
setup. Alternative 3 is probably the preferred option but would be a
more involved change.

This CL takes path 4. It's not an optimal solution, but it can be
replace by alternative 3 later.

TBR=ulan@chromium.org

Bug: v8:7718, v8:7751
Change-Id: I36c024cb0179615011c886ed3978bc95f0d197ac
Reviewed-on: https://chromium-review.googlesource.com/1098924Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53886}