Bug 1993379 part 2: Add "Get" prefix to various SVGTextFrame iterator methods that can return nullptr. r=longsonr,firefox-svg-reviewers

This doesn't change behavior; it's just a function rename. All of these
functions are allowed to return nullptr, so we include "Get" in the name as a
clue about that.

Note that some of the callsites do already include a nullcheck; others do not.
Presumably/hopefully those that don't null-check are downstream of earlier
checks that ensure that the pointer is non-null.  (Or if they're not, then the
new method-names will give us a better clue about the need for a null check, if
we find ways to trigger a null-deref.)

Differential Revision: https://phabricator.services.mozilla.com/D268058

Author: dholbert