Clarify comment in SingleThreadTaskRunner.java

Bug: 863341, 872372
Change-Id: I19405b9061b3dc677d1f06e16564eb9aed133f79
Reviewed-on: https://chromium-review.googlesource.com/1251546
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595569}

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

@@ -5,6 +5,9 @@
 package org.chromium.base.task;
 
 /**
- * Tasks posted will be run in order on a single thread.
+ * Tasks posted will be run in order on a single thread. Multiple SingleThreadTaskRunners
+ * can share a single thread. When sharing a thread, mutual exclusion is guaranteed but
+ * unless specified otherwise by the provider of a given SingleThreadTaskRunner there are
+ * no ordering guarantees w.r.t. other SingleThreadTaskRunner.
  */
 public interface SingleThreadTaskRunner extends SequencedTaskRunner {}