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This lets us remove ambient replace-as-executable from things that don't
use V8.  This includes:

* cast_runner and web_runner, which simply provide fuchsia.sys.Runner
  glue implementations
* http, which provides a legacy HTTP implementation that also doesn't
  need to directly execute memory.

We can also remove the implied executability from some tests.  As a
start, convert the base unittests (which test LoadLibrary) to eschew
the deprecated-ambient-replace-as-executable feature.

Bug: fxb/37924
Change-Id: I461312972881a0e05d31834227e482c78b445ef9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881697
Commit-Queue: Drew Fisher <zarvox@google.com>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711367}