[CSS Grid Layout] Do log(n) search in the named line vectors when positioning named line spans

Implement the suggested FIXMEs and do a log search in the named line
vectors. This maintains the previous (somewhat tricky) behavior by
using std::lower_bound and std::upper_bound. No difference in existing
performance tests, but should scale much better for big grids.

BUG=303217

Review URL: https://codereview.chromium.org/28823002

git-svn-id: svn://svn.chromium.org/blink/trunk@160187 bbb929c8-8fbe-4397-9dbb-9b2b20218538