Use string.substr instead of string.replace to save a copy

Pass by constref and only copy up to the first 1000 characters, rather
than copying it all and inserting a nullptr and ellipsis

Also removed the old definition of kMaxUrlLength from media_log.h and
the old truncate method definition

Bug: 1090752
Change-Id: I6665f010937c802b94da9fd344ab3baf6af88a99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299195
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788846}

media/base/media_log.h

@@ -180,16 +180,6 @@ class MEDIA_EXPORT MediaLog {
   // Helper methods to create events and their parameters.
   std::unique_ptr<MediaLogRecord> CreateRecord(MediaLogRecord::Type type);
 
-  enum : size_t {
-    // Max length of URLs in Created/Load events. Exceeding triggers truncation.
-    kMaxUrlLength = 1000,
-  };
-
-  // URLs (for Created and Load events) may be of arbitrary length from the
-  // untrusted renderer. This method truncates to |kMaxUrlLength| before storing
-  // the event, and sets the last 3 characters to an ellipsis.
-  static std::string TruncateUrlString(std::string log_string);
-
   // The underlying media log.
   scoped_refptr<ParentLogRecord> parent_log_record_;
 

media/base/media_log_events.cc

@@ -41,11 +41,11 @@ std::string MediaLogEventToString(MediaLogEvent level) {
   return "";
 }
 
-std::string TruncateUrlString(std::string url) {
-  constexpr size_t kMaxUrlLength = 1000;
+std::string TruncateUrlString(const std::string& url) {
   if (url.length() > kMaxUrlLength) {
-    url.resize(kMaxUrlLength);
-    url.replace(url.end() - 3, url.end(), "...");
+    // Take substring and _then_ replace, to avoid copying unused data.
+    return url.substr(0, kMaxUrlLength)
+        .replace(kMaxUrlLength - 3, kMaxUrlLength, "...");
   }
   return url;
 }

media/base/media_log_events.h

@@ -12,6 +12,9 @@
 
 namespace media {
 
+// Maximum length of urls that we can show.
+constexpr size_t kMaxUrlLength = 1000;
+
 // Events are changes in the state of a player, or a user interaction, or any
 // other internal representation of a player at a given point in time.
 // This list contains both events that are instant, such as play/pause, as
@@ -68,7 +71,7 @@ enum class MediaLogEvent {
 MEDIA_EXPORT std::string MediaLogEventToString(MediaLogEvent level);
 
 // Sometimes URLs can have encoded data that can be exteremly large.
-MEDIA_EXPORT std::string TruncateUrlString(std::string url);
+MEDIA_EXPORT std::string TruncateUrlString(const std::string& url);
 
 // These events can be triggered with no extra associated data.
 MEDIA_LOG_EVENT_TYPELESS(kPlay);