This was misleading. The spec just wants us to check a string matches
a string in the JavaScript MIME type essence list. It doesn't want us
to parse the string as a MIME type to then use its essence for the
check.
Renames "mime_type" to "string" to make this less misleading.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
This fills in a bunch of the FIXMEs that was in prepare_script.
execute_script is almost finished, it's just missing the module side.
As an aside, let's not assert when inserting a script element with
innerHTML.
