Apply minimum content length check for Readability distillation only.

Since the Reading List feature on iOS continues to use DOM distiller
for distillation, this change minimizes the scope of the new
Readability feature by only applying it to the intended use case.

Fixed: 443075181, 436137793
Change-Id: Iede7913747ad664f57d61eba86d80416bd308341
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6940233
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@google.com>
Cr-Commit-Position: refs/heads/main@{#1514851}