Use pickle for field trial entries in shared memory

This CL changes the data format we use to store field trials in shared
memory from custom offsets to a pickled structure, which has several
advantages. Namely, it handles offsets for us and requires less code.
We will also probably end up using this for experiment parameters as
well.

BUG=660128

Review-Url: https://codereview.chromium.org/2449783007
Cr-Commit-Position: refs/heads/master@{#428423}
1 file changed