[PartitionAlloc] Don't post several purge tasks at the same time.

ThreadCacheRegistry::StartPeriodicPurge() can be called several times,
triggering a DCHECK(), or posting multiple tasks on non-DCHECK()
builds. This happens in particular when the browser is first started in
minimal mode, then in regular mode on Android. Make sure that only one
task is posted.

Issue discovered by tasak@.

Bug: 998048
Change-Id: I7cea58fa0f49487504869f87fec8490e04616aee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613840Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842471}

base/allocator/partition_allocator/thread_cache.cc

@@ -106,7 +106,9 @@ void ThreadCacheRegistry::PurgeAll() {
 }
 
 void ThreadCacheRegistry::StartPeriodicPurge() {
-  PostDelayedPurgeTask();
+  // Can be called several times, don't post multiple tasks.
+  if (!has_pending_purge_task_)
+    PostDelayedPurgeTask();
 }
 
 void ThreadCacheRegistry::PostDelayedPurgeTask() {