Refactor SetOmahaExperimentLabel out of gcpai and into install_util.

chrome/installer/util/google_update_constants.cc

Index: chrome/installer/util/google_update_constants.cc
diff --git a/chrome/installer/util/google_update_constants.cc b/chrome/installer/util/google_update_constants.cc
index 4ab996914e9920cba7b5de437fad08aaf3be397c..c65a014d20bb0104d55ef5ecdb5f9420409fc1be 100644
--- a/chrome/installer/util/google_update_constants.cc
+++ b/chrome/installer/util/google_update_constants.cc
@@ -7,6 +7,9 @@
 namespace google_update {
 
 const wchar_t kChromeUpgradeCode[] = L"{8A69D345-D564-463C-AFF1-A69D9E530F96}";
+
+const wchar_t kExperimentLabels[] = L"experiment_labels";
+
 const wchar_t kGoogleUpdateUpgradeCode[] =
     L"{430FD4D0-B729-4F61-AA34-91526481799D}";