Script position calculation logic (i.e. line & column numbers for a
given code position) is now based on a single method
Script::GetPositionInfo(). Refactored related code in isolate.cc and
js/messages.js to use the new method. The line_ends accessor is still
in use by chromium and thus cannot be removed yet.

R=yangguo@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2002993002
Cr-Commit-Position: refs/heads/master@{#36458}