Move EnumerateModulesModel work to the blocking pool.

chrome/browser/win/enumerate_modules_model.cc

Index: chrome/browser/win/enumerate_modules_model.cc
diff --git a/chrome/browser/win/enumerate_modules_model.cc b/chrome/browser/win/enumerate_modules_model.cc
index ef0e556cdb2132e5572761c97e410005e45c05ce..4b64f86030b5113aeebd201b6b27385da2c42448 100644
--- a/chrome/browser/win/enumerate_modules_model.cc
+++ b/chrome/browser/win/enumerate_modules_model.cc
@@ -376,10 +376,15 @@ void ModuleEnumerator::ScanNow(ModulesVector* list) {
   enumerated_modules_ = list;
 
   // This object can't be reaped until it has finished scanning, so its safe
-  // to post a raw pointer to another thread.
-  BrowserThread::PostTask(BrowserThread::FILE, FROM_HERE,
-                          base::Bind(&ModuleEnumerator::ScanImpl,
-                                     base::Unretained(this)));
+  // to post a raw pointer to another thread. It will simply be leaked if the
+  // scanning has not been finished before shutdown.
+  auto* pool = BrowserThread::GetBlockingPool();
+  auto token = pool->GetSequenceToken();
+  auto runner = pool->GetSequencedTaskRunnerWithShutdownBehavior(
+      token, base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN);
+  runner->PostTask(FROM_HERE,

[gab, 2016/09/26 19:37:21]

Since you don't need a SequencedTaskRunner for multiple tasks you can use
SequencedWorkerPool::PostWorkerTaskWithShutdownBehavior() directly without
getting a temporary SequencedTaskRunner 

[chrisha, 2016/09/26 19:58:17]

Ah, even better, thanks.

+                   base::Bind(&ModuleEnumerator::ScanImpl,
+                              base::Unretained(this)));
 }
 
 void ModuleEnumerator::ScanImpl() {
@@ -721,10 +726,18 @@ void EnumerateModulesModel::MaybePostScanningTask() {
 void EnumerateModulesModel::ScanNow() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  // If a module enumerator exists then a scan is already underway.
+  // If a module enumerator exists then a scan is already underway. The client
+  // will receive a notification from that thread.
   if (module_enumerator_)
     return;
 
+  // Only allow a single scan per process lifetime. Immediately notify any
+  // observers that the scan is complete.
+  if (!enumerated_modules_.empty()) {
+    FOR_EACH_OBSERVER(Observer, observers_, OnScanCompleted());
+    return;
+  }

[gab, 2016/09/26 19:37:21]

I'm unclear on why this change is required/related?

[chrisha, 2016/09/26 19:58:17]

Oops, that was meant to be another CL. Will move it there.

+
   // ScanNow does not block, rather it simply schedules a task.
   module_enumerator_.reset(new ModuleEnumerator(this));
   module_enumerator_->ScanNow(&enumerated_modules_);