Remove dependency on base::SequencedWorkerPool

ImageFetcher only require that all the task be eventually executed
on any thread, in any order so it does no need to depends on the
implementation class base::SequencedWorkerPool but can work with
just the interface base::TaskRunner.

BUG=None

Review URL: https://codereview.chromium.org/808833004

Cr-Commit-Position: refs/heads/master@{#308825}

ios/chrome/browser/net/image_fetcher.h

@@ -17,7 +17,7 @@ class GURL;
 @class NSData;
 
 namespace base {
-class SequencedWorkerPool;
+class TaskRunner;
 }
 
 namespace net {
@@ -40,9 +40,8 @@ using ImageFetchedCallback =
 // An instance of this class can download a number of images at the same time.
 class ImageFetcher : public net::URLFetcherDelegate {
  public:
-  // The WorkerPool is used to eventually decode the image.
-  explicit ImageFetcher(
-      const scoped_refptr<base::SequencedWorkerPool>& decoding_pool);
+  // The TaskRunner is used to eventually decode the image.
+  explicit ImageFetcher(const scoped_refptr<base::TaskRunner>& task_runner);
   ~ImageFetcher() override;
 
   // Start downloading the image at the given |url|. The |callback| will be
@@ -86,8 +85,8 @@ class ImageFetcher : public net::URLFetcherDelegate {
   // during WebP decoding.
   base::WeakPtrFactory<ImageFetcher> weak_factory_;
 
-  // The pool used to decode images if necessary.
-  const scoped_refptr<base::SequencedWorkerPool> decoding_pool_;
+  // The task runner used to decode images if necessary.
+  const scoped_refptr<base::TaskRunner> task_runner_;
 };
 
 }  // namespace image_fetcher

ios/chrome/browser/net/image_fetcher.mm

@@ -9,7 +9,7 @@
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/mac/scoped_nsobject.h"
-#include "base/threading/sequenced_worker_pool.h"
+#include "base/task_runner.h"
 #include "ios/web/public/web_thread.h"
 #include "ios/web/public/webp_decoder.h"
 #include "net/base/load_flags.h"
@@ -60,12 +60,11 @@ void OnDataDecoded(NSData* data) override {
 
 namespace image_fetcher {
 
-ImageFetcher::ImageFetcher(
-    const scoped_refptr<base::SequencedWorkerPool>& decoding_pool)
+ImageFetcher::ImageFetcher(const scoped_refptr<base::TaskRunner>& task_runner)
     : request_context_getter_(nullptr),
       weak_factory_(this),
-      decoding_pool_(decoding_pool) {
-  DCHECK(decoding_pool_.get());
+      task_runner_(task_runner) {
+  DCHECK(task_runner_.get());
 }
 
 ImageFetcher::~ImageFetcher() {
@@ -148,7 +147,7 @@ void OnDataDecoded(NSData* data) override {
     std::string mime_type;
     fetcher->GetResponseHeaders()->GetMimeType(&mime_type);
     if (mime_type == kWEBPMimeType) {
-      base::PostTaskAndReplyWithResult(decoding_pool_.get(),
+      base::PostTaskAndReplyWithResult(task_runner_.get(),
                                        FROM_HERE,
                                        base::Bind(&DecodeWebpImage, data),
                                        base::Bind(&ImageFetcher::RunCallback,