Rework field_trial.h comment.
Rework field_trial.h comment.

This comment was getting increasingly out of date; the interfaces were
wrong, and the coding practices suggested wouldn't get past review.

Here's my first pass at fixing the comment so it at least will compile and
pass a review.  Have at it!

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6685031

git-svn-id: http://src.chromium.org/svn/trunk/src@78879 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 8b076bdd7bf8797818cfe3900909490ec0562efe
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser