There was previously a race between different phases recording their
first entry -- and thus, their insert order -- on the main and
concurrent-compilation thread. This would later manifest as a segfault
when creating the sorted array of phases for --turbo-stats (as two
phases would have the same insert order and so there would be a gap in
the array).

Review-Url: https://codereview.chromium.org/2572713003
Cr-Commit-Position: refs/heads/master@{#41669}