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* Fix the implementation of has_single_url() method.
It is incorrect to check is_valid() because the elements
vector can contain more than 1 item. And thus the
is_valid() check does not make sense, since it just check
if the vector is not empty, not that it just contains one element.

* Use is_valid() method when possible.
* Use has_single_url() method when it makes sense.
* Use data.size() to avoid a temp variable.

BUG=None
TEST=chrome still links and works as before, existing unit tests in components_unittests.
R=sky@chromium.org

Review URL: https://codereview.chromium.org/870293002

Cr-Commit-Position: refs/heads/master@{#313702}