Refactor BreakLocationIterator.

Refactor BreakLocationIterator.

We now have BreakLocation::Iterator to iterate via RelocIterator, and
create a BreakLocation when we are done iterating. The reloc info is
stored in BreakLocation in a GC-safe way and instantiated on demand.

R=ulan@chromium.org
BUG=v8:3924
LOG=N

Review URL: https://codereview.chromium.org/967323002

Cr-Commit-Position: refs/heads/master@{#26983}