AutoArraySchemaWriter::FreeFormElement() is never used, we can remove it.

When constructed, AutoArraySchemaWriter can optionally take a UniqueStrings reference, which is stored as a pointer.
Then AutoArraySchemaWriter::StringElement() does a MOZ_RELEASE_ASSERT(mStrings);.

We should split the class into two:

• A writer that does not deal with strings at all, so we don't need a pointer to UniqueStrings.
• A writer that can also deal with strings, in which we store a (non-null) UniqueStrings reference.

This is both:

• Safer, because it's not possible to instantiate the non-string writer and try to write a string.
• More efficient, because we don't need to pass&store a UniqueStrings reference/pointer when we don't deal with strings.

(Found while working on bug 1329600, in which it will be quite useful.)

AutoArraySchemaWriter::FreeFormElement() is never used, we can remove it.

When constructed, AutoArraySchemaWriter was optionally taking a UniqueStrings reference, which was stored as a pointer.
Then AutoArraySchemaWriter::StringElement() would do a dangerous MOZ_RELEASE_ASSERT(mStrings);.

The class is now split in two:

• AutoArraySchemaWriter, which does not deal with strings at all, so we don't need a pointer to UniqueStrings.
• AutoArraySchemaWithStringsWriter that can also deal with strings, in which we store a (non-null) UniqueStrings reference.

This is both:

• Safer, because it's not possible to instantiate the non-string writer and try to write a string.
• More efficient, because we don't need to pass&store a UniqueStrings reference/pointer when we don't deal with strings.

https://hg.mozilla.org/mozilla-central/rev/c19042d45111