[Android] Record UMA metrics after native initialization completes.

Discoverd that we never got LibraryLoader.NativeLibraryHack in UMA
histograms server. Found that's because UMA session was not started
yet right after native libraries are loaded.

By delaying recording until UMA session is started, these stats
shown up in chrome://histograms.

BUG=311644

Review URL: https://codereview.chromium.org/452923004

Cr-Commit-Position: refs/heads/master@{#289022}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289022 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -225,7 +225,10 @@ public class LibraryLoader {
 
         // From now on, keep tracing in sync with native.
         TraceEvent.registerNativeEnabledObserver();
+    }
 
+    // Called after all native initializations are complete.
+    public static void onNativeInitializationComplete() {
         // Record histogram for the Chromium linker.
         if (Linker.isUsed()) {
             nativeRecordChromiumAndroidLinkerHistogram(Linker.loadAtFixedAddressFailed(),