Add metric to report number of URL switches per update attempt

BUG=chromium:226765
TEST=Existing unit tests modified to test feature, Unit tests pass

Change-Id: I5ee0816cac3fa4fa641949ed4872aef8074e3fcc
Reviewed-on: https://gerrit.chromium.org/gerrit/48537
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/constants.h b/constants.h
index b2c1de2..3b8a918 100644
--- a/constants.h
+++ b/constants.h
@@ -32,6 +32,7 @@
 extern const char kPrefsTotalBytesDownloaded[];
 extern const char kPrefsUpdateCheckCount[];
 extern const char kPrefsUpdateCheckResponseHash[];
+extern const char kPrefsUpdateDurationUptime[];
 extern const char kPrefsUpdateFirstSeenAt[];
 extern const char kPrefsUpdateServerCertificate[];
 extern const char kPrefsUpdateStateNextDataOffset[];
@@ -39,9 +40,9 @@
 extern const char kPrefsUpdateStateSHA256Context[];
 extern const char kPrefsUpdateStateSignatureBlob[];
 extern const char kPrefsUpdateStateSignedSHA256Context[];
-extern const char kPrefsWallClockWaitPeriod[];
 extern const char kPrefsUpdateTimestampStart[];
-extern const char kPrefsUpdateDurationUptime[];
+extern const char kPrefsUrlSwitchCount[];
+extern const char kPrefsWallClockWaitPeriod[];
 
 // A download source is any combination of protocol and server (that's of
 // interest to us when looking at UMA metrics) using which we may download