Stop fetching the next page if the first page has no content

Stop fetching the next page if the first page has no content

In DOM distiller, if the distilled content of the first page is empty,
it's very likely that the page is not good for distillation, so fetching
the next page doesn't make sense.

BUG=602139
Review URL: https://codereview.chromium.org/1891103002

Cr-Commit-Position: refs/heads/master@{#387759}
(cherry picked from commit 6d72911b0cb47f5519288325fc502470815b9e7a)

[wychen, 2016-04-20 19:22:35]

Committed patchset #1 (id:1) to pending queue manually as
04991e720249701a0020a334a554f4c614eab338.