Separate version env variable for different distributions.

chrome/installer/util/google_chrome_distribution.cc

Index: chrome/installer/util/google_chrome_distribution.cc
===================================================================
--- chrome/installer/util/google_chrome_distribution.cc	(revision 48325)
+++ chrome/installer/util/google_chrome_distribution.cc	(working copy)
@@ -479,6 +479,10 @@
   return key;
 }
 
+std::wstring GoogleChromeDistribution::GetEnvVersionKey() {
+  return L"CHROME_VERSION";
+}
+
 // This method checks if we need to change "ap" key in Google Update to try
 // full installer as fall back method in case incremental installer fails.
 // - If incremental installer fails we append a magic string ("-full"), if