[bootstrapper] Use InternalizeUtf8String more consistently

Previously, bootstrapper.cc contained a mixture of approaches:

- NewStringFromAsciiChecked("foo"): 40 matches
- NewStringFromStaticChars("foo"): 4 matches
- InternalizeUtf8String("foo"): 55 matches

The most common use case for any of these in the bootstrapper is
to represent property names. For those, we eventually need internalized
strings anyhow. E.g. NewStringFromAscii causes an InternalizeString
call later, possibly creating a copy or ThinString.

This patch uses InternalizeUtf8String where it makes sense to do so.

https://chromium-review.googlesource.com/c/v8/v8/+/1253603/1/src/bootstrapper.cc#2098

Bug: v8:8238
Change-Id: I124607988b75449d7f78d5933657c35b532bd1c9
Reviewed-on: https://chromium-review.googlesource.com/c/1255727
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56417}