Correct ParagraphBuilder::getText() behavior

And make sure that no text modifications could be done
after ParagraphBuilder::Build() until Reset()

Change-Id: I5e7b2a99fb4577ddd41e23d153783642d606aaef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/618518
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
2 files changed