Relands 5b41612e, reverted at 0b24f770.

Breakpad was being synced from a read-only mirror of a subdirectory
rather than the authoritative upstream source. This encouraged Chrome
developers to send patches against the read-only mirror. Those patches
couldn’t be committed and needed to be duplicated in the correct
repository. Once, a patch against the read-only mirror accidentally was
committed, and it broke mirroring.

This odd setup existed from the time of the git migration. Prior to
that, Breakpad was hosted in Subversion, and Chrome incidentally pulled
Breakpad’s src subdirectory rather than the entire trunk directory.
Because it wasn‘t easy to do the same thing with git, the read-only
mirror of src was created to avoid having to change lots of paths in
Chrome. But now I’m changing the paths.

This also moves Breakpad from the top-level breakpad directory in Chrome
to third_party/breakpad. It should have been there...