When persisting UMA logs to disk, cap log size based on both the XML and protocol buffer logs.

chrome/common/metrics/metrics_log_manager.h

Index: chrome/common/metrics/metrics_log_manager.h
diff --git a/chrome/common/metrics/metrics_log_manager.h b/chrome/common/metrics/metrics_log_manager.h
index 982332225eb444e47a27f7117e0662f354ec5df4..d2761946841f000c93803872ed0073e5a8eb8f44 100644
--- a/chrome/common/metrics/metrics_log_manager.h
+++ b/chrome/common/metrics/metrics_log_manager.h
@@ -28,6 +28,7 @@ class MetricsLogManager {
     // Exposed to reduce code churn as we transition from the XML pipeline to
     // the protocol buffer pipeline.
     bool empty() const;
+    size_t length() const;
     void swap(SerializedLog& log);
 
     std::string xml;