Early return if we find selected shipping option from last.

The spec requires using the last selected shipping option.
Instead of going through the whole list of shipping options
from front to back, iterate from back to front and early
return on the first selected shipping option.

BUG=587995
TEST=None (No behavior change)

Review-Url: https://codereview.chromium.org/2131963002
Cr-Commit-Position: refs/heads/master@{#404449}
1 file changed