Revert of Use TaskScheduler instead of SequencedWorkerPool in snapsho…

…t_cache.mm. (patchset #2 id:20001 of https://codereview.chromium.org/2881873002/ )

Reason for revert:
Probably this CL causes many failures of iOS bots. For example, https://build.chromium.org/p/chromium.mac/builders/ios-simulator/builds/17573

Original issue's description:
> Use TaskScheduler instead of SequencedWorkerPool in snapshot_cache.mm.
>
> SequencedWorkerPool is being deprecated in favor of TaskScheduler.
>
> BUG=667892
> R=jif@chromium.org
>
> Review-Url: https://codereview.chromium.org/2881873002
> Cr-Commit-Position: refs/heads/master@{#472052}
> Committed: https://chromium.googlesource.com/chromium/src/+/aa5299ae23a57ca5dce00a11da3ac8e73849351f

TBR=jif@chromium.org,fdoray@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=667892

Review-Url: https://codereview.chromium.org/2889553002
Cr-Commit-Position: refs/heads/master@{#472065}

ios/chrome/browser/snapshots/snapshot_cache.mm

@@ -18,7 +18,6 @@
 #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"
@@ -575,10 +574,9 @@ - (void)saveGreyInBackgroundForSessionID:(NSString*)sessionID {
     }
   }
 
-  base::PostTaskWithTraits(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},
-      base::BindOnce(&ConvertAndSaveGreyImage, colorImagePath, greyImagePath,
-                     backgroundingColorImage_));
+  web::WebThread::PostBlockingPoolTask(
+      FROM_HERE, base::Bind(&ConvertAndSaveGreyImage, colorImagePath,
+                            greyImagePath, backgroundingColorImage_));
 }
 
 @end