Bug 1594820: Part 4 - Modify GeckoThread.preload to directly call into the process manager; r=geckoview-reviewers,snorp

Now that everything in `GeckoProcessManager` runs on the XPCOM launcher thread,
`GeckoThread` should just call `GeckoProcessManager.preload()` directly.

Differential Revision: https://phabricator.services.mozilla.com/D57840

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/gecko/GeckoThread.java

@@ -448,13 +448,8 @@ public class GeckoThread extends Thread {
         initGeckoEnvironment();
 
         if ((mInitInfo.flags & FLAG_PRELOAD_CHILD) != 0) {
-            ThreadUtils.postToBackgroundThread(new Runnable() {
-                @Override
-                public void run() {
-                    // Preload the content ("tab") child process.
-                    GeckoProcessManager.getInstance().preload("tab");
-                }
-            });
+            // Preload the content ("tab") child process.
+            GeckoProcessManager.getInstance().preload("tab");
         }
 
         if ((mInitInfo.flags & FLAG_DEBUGGING) != 0) {