GCAPI should append to the existing experiment_labels instead of clobbering them.

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 9f516ea5e132767860faa28ad9e7f61f24ca11c5..d17bfbb2e554e585b037418c6c9c826ad8b459e7 100644
--- a/chrome/installer/util/google_update_constants.cc
+++ b/chrome/installer/util/google_update_constants.cc
@@ -9,6 +9,7 @@ namespace google_update {
 const wchar_t kChromeUpgradeCode[] = L"{8A69D345-D564-463C-AFF1-A69D9E530F96}";
 
 const wchar_t kExperimentLabels[] = L"experiment_labels";
+const wchar_t kExperimentLabelSep[] = L";";
 
 const wchar_t kGoogleUpdateUpgradeCode[] =
     L"{430FD4D0-B729-4F61-AA34-91526481799D}";