Here's my plan:

  - Create a base/metrics/field_trial_params.[cc|h] API that mimics the params APIs in components/variations/variations_associated_data.h - basically moving those implementations and renaming them.
  - Mark the APIs in variations_associated_data.h as deprecated with a note to use the base/ versions. Make those APIs just call through to the new base ones.
  - At this point, people can now start using params from subprocesses.
  - After some time, migrate remaining callers of the variations APIs to the base ones and delete the variations ones.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79ab08c0137f5485173e8de5cd28201774ce1a0f

commit 79ab08c0137f5485173e8de5cd28201774ce1a0f
Author: asvitkine <asvitkine@chromium.org>
Date: Mon Jan 30 23:27:05 2017

Move API for field trial params to base from variations.

Now that the underlying implementation also works in sub-processes,
this CL moves the APIs to base (naming the field trial params), so
that they can be easily accessed by all Chrome sub-processes.

Marks the existing variation params APIs as deprecated. Existing
callers can be migrates via future CLs and these can then be
removed.

Unit tests also moved to base.

BUG= 681160 

Review-Url: https://codereview.chromium.org/2667553002
Cr-Commit-Position: refs/heads/master@{#447108}

[modify] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/base/BUILD.gn
[add] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/base/metrics/field_trial_params.cc
[add] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/base/metrics/field_trial_params.h
[add] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/base/metrics/field_trial_params_unittest.cc
[modify] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/components/variations/variations_associated_data.cc
[modify] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/components/variations/variations_associated_data.h
[modify] https://crrev.com/79ab08c0137f5485173e8de5cd28201774ce1a0f/components/variations/variations_associated_data_unittest.cc

Fixed!

It should be possible to use variation params (now field trial params) from subprocesses now.

Can this be merged to M57?

I just landed https://codereview.chromium.org/2657923002 which needs to be merged back to 57, but realized I built it on top of the API added in this change.

It should be possible to merge if we also merge the dependent change here:
https://codereview.chromium.org/2633203002

However, since you're just using the API from browser process, I suggest to just use the components/variations version of the API when you merge your CL and then you don't need to merge this change.

Ok, yea switching my change to the old API is probably a cleaner route than merging the API move in the change here back to 57.

I had to at least ask though. :)  Thanks!