port 1a608493 (r26983)

original commit message:

   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.

BUG=

Review URL: https://codereview.chromium.org/978183002

Cr-Commit-Position: refs/heads/master@{#27003}