[reland] Use TaskScheduler instead of SequencedWorkerPool in snapshot_cache.mm.

ios/chrome/browser/snapshots/snapshot_cache.mm

Index: ios/chrome/browser/snapshots/snapshot_cache.mm
diff --git a/ios/chrome/browser/snapshots/snapshot_cache.mm b/ios/chrome/browser/snapshots/snapshot_cache.mm
index 7fb720d8c21ab9c55791dd7c850208a8d56a891a..612dc57e046e3a40fc50afe8e6b9917b34309fb2 100644
--- a/ios/chrome/browser/snapshots/snapshot_cache.mm
+++ b/ios/chrome/browser/snapshots/snapshot_cache.mm
@@ -10,6 +10,7 @@
 #include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
+#include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/mac/bind_objc_block.h"
@@ -18,6 +19,7 @@
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/task_runner_util.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_restrictions.h"
 #include "ios/chrome/browser/experimental_flags.h"
 #import "ios/chrome/browser/snapshots/lru_cache.h"
@@ -52,12 +54,20 @@ static NSArray* const kSnapshotCacheDirectory = @[ @"Chromium", @"Snapshots" ];
 const NSUInteger kCacheInitialCapacity = 100;
 const NSUInteger kGreyInitialCapacity = 8;
 const CGFloat kJPEGImageQuality = 1.0;  // Highest quality. No compression.
-// Sequence token to make sure creation/deletion of snapshots don't overlap.
-const char kSequenceToken[] = "SnapshotCacheSequenceToken";
 // Maximum size in number of elements that the LRU cache can hold before
 // starting to evict elements.
 const NSUInteger kLRUCacheMaxCapacity = 6;
 
+struct SnapshotTaskRunner {
+  const scoped_refptr<base::SequencedTaskRunner> task_runner =
+      base::CreateSequencedTaskRunnerWithTraits(
+          {base::MayBlock(), base::TaskPriority::USER_VISIBLE});
+};
+
+// Sequence token to make sure creation/deletion of snapshots don't overlap.
+base::LazyInstance<SnapshotTaskRunner>::Leaky g_snapshot_task_runner =
+    LAZY_INSTANCE_INITIALIZER;
+
 // The paths of the images saved to disk, given a cache directory.
 base::FilePath FilePathForSessionID(NSString* sessionID,
                                     const base::FilePath& directory) {
@@ -275,9 +285,8 @@ void ConvertAndSaveGreyImage(
     [imageDictionary_ setObject:img forKey:sessionID];
 
   // Save the image to disk.
-  web::WebThread::PostBlockingPoolSequencedTask(
-      kSequenceToken, FROM_HERE,
-      base::BindBlock(^{
+  g_snapshot_task_runner.Get().task_runner->PostTask(
+      FROM_HERE, base::BindBlock(^{
         base::scoped_nsobject<UIImage> image([img retain]);
         WriteImageToDisk(image,
                          [SnapshotCache imagePathForSessionID:sessionID]);
@@ -291,9 +300,8 @@ void ConvertAndSaveGreyImage(
   else
     [imageDictionary_ removeObjectForKey:sessionID];
 
-  web::WebThread::PostBlockingPoolSequencedTask(
-      kSequenceToken, FROM_HERE,
-      base::BindBlock(^{
+  g_snapshot_task_runner.Get().task_runner->PostTask(
+      FROM_HERE, base::BindBlock(^{
         base::FilePath imagePath =
             [SnapshotCache imagePathForSessionID:sessionID];
         base::DeleteFile(imagePath, false);
@@ -357,9 +365,8 @@ void ConvertAndSaveGreyImage(
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   // Copying the date, as the block must copy the value, not the reference.
   const base::Time dateCopy = date;
-  web::WebThread::PostBlockingPoolSequencedTask(
-      kSequenceToken, FROM_HERE,
-      base::BindBlock(^{
+  g_snapshot_task_runner.Get().task_runner->PostTask(
+      FROM_HERE, base::BindBlock(^{
         std::set<base::FilePath> filesToKeep;
         for (NSString* sessionID : liveSessionIds) {
           base::FilePath curImagePath =
@@ -574,9 +581,10 @@ void ConvertAndSaveGreyImage(
     }
   }
 
-  web::WebThread::PostBlockingPoolTask(
-      FROM_HERE, base::Bind(&ConvertAndSaveGreyImage, colorImagePath,
-                            greyImagePath, backgroundingColorImage_));
+  base::PostTaskWithTraits(
+      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},
+      base::BindOnce(&ConvertAndSaveGreyImage, colorImagePath, greyImagePath,
+                     backgroundingColorImage_));
 }
 
 @end