PathService::Get() is treated as safe-to-call from any thread, but
PathProviderMac(FILE_EXE,...) looks up the NSExecutable path, and
translates it from relative to absolute using
base::MakeAbsoluteFilePath(), which asserts that IO is allowed on the
calling thread.

This was being hit by code in NaClBrowser in developers' local debug
builds.

As per the discussion on crbug.com/259796, the one-off cost of this
IO (given PathService then caches the result) seems acceptable.

NOPRESUBMIT=true
BUG=373477, 259796

Review-Url: https://codereview.chromium.org/2145003004
Cr-Commit-Position: refs/heads/master@{#405577}