Move SafeBrowsing to the blocking pool via an experiment.

chrome/browser/safe_browsing/safe_browsing_store_file.cc

Index: chrome/browser/safe_browsing/safe_browsing_store_file.cc
diff --git a/chrome/browser/safe_browsing/safe_browsing_store_file.cc b/chrome/browser/safe_browsing/safe_browsing_store_file.cc
index 47c3f9efafd73e8fb954b0a5130230f3fd2f2b0b..358ecf684e754b115dff07d7d37cd4b24e91b264 100644
--- a/chrome/browser/safe_browsing/safe_browsing_store_file.cc
+++ b/chrome/browser/safe_browsing/safe_browsing_store_file.cc
@@ -554,16 +554,25 @@ bool ReadDbStateHelper(const base::FilePath& filename,
 
 }  // namespace
 
-SafeBrowsingStoreFile::SafeBrowsingStoreFile()
-    : chunks_written_(0), empty_(false), corruption_seen_(false) {}
+SafeBrowsingStoreFile::SafeBrowsingStoreFile(
+    scoped_refptr<base::SequencedTaskRunner> task_runner)
+    : task_runner_(task_runner),
+      chunks_written_(0),
+      empty_(false),
+      corruption_seen_(false) {
+}
 
 SafeBrowsingStoreFile::~SafeBrowsingStoreFile() {
   // Thread-checking is disabled in the destructor due to crbug.com/338486.
-  DetachFromThread();
+  task_runner_ = nullptr;
 
   Close();
 }
 
+bool SafeBrowsingStoreFile::CalledOnValidThread() {
+  return !task_runner_ || task_runner_->RunsTasksOnCurrentThread();
+}
+
 bool SafeBrowsingStoreFile::Delete() {
   DCHECK(CalledOnValidThread());