Change default Omaha installsource param from "" to "ondemand"

This will let Omaha know the difference between new, on-demand installs
that are manually initiated from the webstore, and the background
autoupdate checks that we do for already installed extensions/apps.

BUG=249187

Review URL: https://chromiumcodereview.appspot.com/17524010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@208048 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/extensions/webstore_installer.cc

@@ -66,7 +66,7 @@ const char kInstallCanceledError[] = "Install canceled";
 const char kDownloadInterruptedError[] = "Download interrupted";
 const char kInvalidDownloadError[] = "Download was not a CRX";
 const char kInlineInstallSource[] = "inline";
-const char kDefaultInstallSource[] = "";
+const char kDefaultInstallSource[] = "ondemand";
 
 base::FilePath* g_download_directory_for_tests = NULL;