Add Installer.UpdatesAbandonedCount metric

This patch adds a new metric Installer.UpdatesAbandonedCount to track
the number of update attempts that didn't complete because a newer
update was detected during the download. This is implemented by
counting the number of different responses seen since the last
successful update.

Updates are typically only abandoned if a device is suspended or
powered off while an update is happening. This can happen either
because the device was not turned on for a very long time or because
it had little or no connectivity to Omaha and/or the servers serving
the payload.

This metric will help show how many users run into this problem.

BUG=chromium:248800
TEST=New units tests + Unit tests pass + Manually tested

Change-Id: I524a380a931c2fb30916d033b7e5b0c700f57103
Reviewed-on: https://gerrit.chromium.org/gerrit/59098
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
diff --git a/constants.h b/constants.h
index e97ec90..4bd9685 100644
--- a/constants.h
+++ b/constants.h
@@ -33,6 +33,7 @@
 extern const char kPrefsLastRollCallPingDay[];
 extern const char kPrefsManifestMetadataSize[];
 extern const char kPrefsNumReboots[];
+extern const char kPrefsNumResponsesSeen[];
 extern const char kPrefsPayloadAttemptNumber[];
 extern const char kPrefsPreviousVersion[];
 extern const char kPrefsResumedUpdateFailures[];