Override unsupported and similar languages for hrefTranslate by default

For hrefTranslate link clicks from Google domains, this CL changes the
default behavior when the detected source language is either unsupported
or similar to the target language. The previous default behavior was to
prevent translation in these cases, which is now changed to attempting
to translate anyways with "und" as the source language, leaving language
detection up to the Translate Service as the translation is performed.

This CL doesn't affect other translation behavior, such as regular
non-hrefTranslate translation in Chrome.

Bug: 903416
Change-Id: Iba8a9f841e7ff8503263795c6d41621ddc0c5e63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3212735
Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/main@{#929762}