In addition to the previous change enabling forced FunctionDeclaration
allocation when block code coverage is enabled, enable it now for all
(non-best-effort) code coverage by reading off the coverage mode from
the isolate (rather than relying on the presence of a source range map).

Bug: chromium:927464
Change-Id: I26f86c9fbebc0df52d5cdeff3ca1095215a6d912
Reviewed-on: https://chromium-review.googlesource.com/c/1456041
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59626}