Move API for field trial params to base from variations.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 7f46e71ae532ed8af7a7c9dfd6873e86672a0a25..718dab6cb01b75146ac32ae9327ee1d1d3b2098c 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -561,6 +561,8 @@ component("base") {
     "metrics/field_trial.h",
     "metrics/field_trial_param_associator.cc",
     "metrics/field_trial_param_associator.h",
+    "metrics/field_trial_params.cc",
+    "metrics/field_trial_params.h",
     "metrics/histogram.cc",
     "metrics/histogram.h",
     "metrics/histogram_base.cc",
@@ -1951,6 +1953,7 @@ test("base_unittests") {
     "message_loop/message_pump_glib_unittest.cc",
     "message_loop/message_pump_io_ios_unittest.cc",
     "metrics/bucket_ranges_unittest.cc",
+    "metrics/field_trial_params_unittest.cc",
     "metrics/field_trial_unittest.cc",
     "metrics/histogram_base_unittest.cc",
     "metrics/histogram_delta_serialization_unittest.cc",