android: Fix PostTask ConcurrentModificationException

Calling initNativeTaskRunner might back into
registerPreNativeTaskRunnerLocked which causes
ConcurrentModificationException in WeakHashMap.

Fix by setting sPreNativeTaskRunners to null first.
onNativeSchedulerReady indicates native side is already ready, so should
be safe to set sPreNativeTaskRunners to null.

Bug: 974674
Change-Id: I77ea44c2b9d8a060c4b60e9c9171d14c91b71f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660971Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669664}

base/android/java/src/org/chromium/base/task/PostTask.java

@@ -239,10 +239,11 @@ public class PostTask {
     @CalledByNative
     private static void onNativeSchedulerReady() {
         synchronized (sLock) {
-            for (TaskRunner taskRunner : sPreNativeTaskRunners) {
+            Set<TaskRunner> preNativeTaskRunners = sPreNativeTaskRunners;
+            sPreNativeTaskRunners = null;
+            for (TaskRunner taskRunner : preNativeTaskRunners) {
                 taskRunner.initNativeTaskRunner();
             }
-            sPreNativeTaskRunners = null;
         }
     }