[Blob] BlobReader class & tests, and removal of all redundant reading.

storage/browser/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP version: storage/storage_browser.gyp:storage
 component("browser") {
   output_name = "storage_browser"
   sources = [
     "blob/blob_data_builder.cc",
     "blob/blob_data_builder.h",
     "blob/blob_data_handle.cc",
     "blob/blob_data_handle.h",
     "blob/blob_data_item.cc",
     "blob/blob_data_item.h",
     "blob/blob_data_snapshot.cc",
     "blob/blob_data_snapshot.h",
+    "blob/blob_reader.cc",
+    "blob/blob_reader.h",
     "blob/blob_storage_context.cc",
     "blob/blob_storage_context.h",
     "blob/blob_url_request_job.cc",
     "blob/blob_url_request_job.h",
     "blob/blob_url_request_job_factory.cc",
     "blob/blob_url_request_job_factory.h",
     "blob/internal_blob_data.cc",
     "blob/internal_blob_data.h",
     "blob/scoped_file.cc",
     "blob/scoped_file.h",
     "blob/shareable_blob_data_item.cc",
     "blob/shareable_blob_data_item.h",
     "blob/shareable_file_reference.cc",
     "blob/shareable_file_reference.h",
+    "blob/upload_blob_element_reader.cc",
+    "blob/upload_blob_element_reader.h",
     "blob/view_blob_internals_job.cc",
     "blob/view_blob_internals_job.h",
     "database/database_quota_client.cc",
     "database/database_quota_client.h",
     "database/database_tracker.cc",
     "database/database_tracker.h",
     "database/database_util.cc",
     "database/database_util.h",
     "database/databases_table.cc",
     "database/databases_table.h",
@@ skipping 162 matching lines @@
     "fileapi/dump_file_system.cc",
   ]
 
   deps = [
     ":browser",
     "//base",
     "//build/config/sanitizers:deps",
     "//storage/common",
   ]
 }