Forward declare base::Time in uniformity trials.

chrome/common/metrics/variations/uniformity_field_trials.cc

Index: chrome/common/metrics/variations/uniformity_field_trials.cc
diff --git a/chrome/common/metrics/variations/uniformity_field_trials.cc b/chrome/common/metrics/variations/uniformity_field_trials.cc
index 600a727b688ddb4d992255e102ab20d18728f564..c7460f069b56d585f11471a0d72016d3ed086066 100644
--- a/chrome/common/metrics/variations/uniformity_field_trials.cc
+++ b/chrome/common/metrics/variations/uniformity_field_trials.cc
@@ -92,7 +92,7 @@ void SetupNewInstallUniformityTrial(const base::Time& install_date) {
 
 namespace chrome_variations {
 
-void SetupUniformityFieldTrials(const base::Time& install_date) {
+void SetupUniformityFieldTrials(const base::Time install_date) {
   // One field trial will be created for each entry in this array. The i'th
   // field trial will have |trial_sizes[i]| groups in it, including the default
   // group. Each group will have a probability of 1/|trial_sizes[i]|.