LibraryLoader: Add residency percentage to trace

This should help triage cases when library prefetching takes too long or
too little.

Bug: 881384
Change-Id: I315020758bb702578237fae6de5c776e27c367d4
Reviewed-on: https://chromium-review.googlesource.com/1215802Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589920}

base/android/java/src/org/chromium/base/TraceEvent.java

@@ -221,7 +221,7 @@ public class TraceEvent implements AutoCloseable {
      * Note that if tracing is not enabled, this will not result in allocating an object.
      *
      * @param name Trace event name.
-     * @param name The arguments of the event.
+     * @param arg The arguments of the event.
      * @return a TraceEvent, or null if tracing is not enabled.
      */
     public static TraceEvent scoped(String name, String arg) {

base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java

@@ -358,8 +358,9 @@ public class LibraryLoader {
 
         @Override
         protected Void doInBackground() {
-            try (TraceEvent e = TraceEvent.scoped("LibraryLoader.asyncPrefetchLibrariesToMemory")) {
-                int percentage = nativePercentageOfResidentNativeLibraryCode();
+            int percentage = nativePercentageOfResidentNativeLibraryCode();
+            try (TraceEvent e = TraceEvent.scoped("LibraryLoader.asyncPrefetchLibrariesToMemory",
+                         Integer.toString(percentage))) {
                 // Arbitrary percentage threshold. If most of the native library is already
                 // resident (likely with monochrome), don't bother creating a prefetch process.
                 boolean prefetch = mColdStart && percentage < 90;