Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/docs/save-page-as.md

 # High-level overview of Save-Page-As code
 
 This document describes code under `//content/browser/downloads`
 restricting the scope only to code handling Save-Page-As functionality
 (i.e. leaving out other downloads-related code).
 This document focuses on high-level overview and aspects of the code that
 span multiple compilation units (hoping that individual compilation units
 are described by their code comments or by their code structure).
 
 ## Classes overview
 
 * SavePackage class
     * coordinates overall save-page-as request
     * created and owned by `WebContents`
       (ref-counted today, but it is unnecessary - see https://crbug.com/596953)
     * UI-thread object
 
 * SaveFileCreateInfo::SaveFileSource enum
     * classifies `SaveItem` and `SaveFile` processing into 2 flavours:
         * `SAVE_FILE_FROM_NET` (see `SaveFileResourceHandler`)
         * `SAVE_FILE_FROM_DOM` (see "Complete HTML" section below)
 
 * SaveItem class
     * tracks saving a single file
     * created and owned by `SavePackage`
     * UI-thread object
 
 * SaveFileManager class
-    * coordinates between FILE and UI threads
+    * coordinates between the download sequence and the UI thread
         * Gets requests from `SavePackage` and communicates results back to
           `SavePackage` on the UI thread.
         * Shephards data (received from the network OR from DOM) into
-          FILE thread - via `SaveFileManager::UpdateSaveProgress`
+          the download sequence - via `SaveFileManager::UpdateSaveProgress`
     * created and owned by `BrowserMainLoop`
       (ref-counted today, but it is unnecessary - see https://crbug.com/596953)
     * The global instance can be retrieved by the Get method.
 
 * SaveFile class
     * tracks saving a single file
     * created and owned by `SaveFileManager`
-    * FILE-thread object
+    * download sequence object
 
 * SaveFileResourceHandler class
     * tracks network downloads + forwards their status into `SaveFileManager`
-      (onto FILE-thread)
+      (onto download sequence)
     * created by `ResourceDispatcherHostImpl::BeginSaveFile`
     * IO-thread object
 
 * SaveFileCreateInfo POD struct
     * short-lived object holding data passed to callbacks handling start of
       saving a file.
 
 * MHTMLGenerationManager class
     * singleton that manages progress of jobs responsible for saving individual
       MHTML files (represented by `MHTMLGenerationManager::Job`).
@@ skipping 75 matching lines @@
 
 * Blink:
     * `//third_party/WebKit/public/web/WebFrameSerializer...`
     * `//third_party/WebKit/Source/web/WebFrameSerializerImpl...`
       (used for Complete HTML today;  should use `FrameSerializer` instead in
       the long-term - see https://crbug.com/328354).
     * `//third_party/WebKit/Source/core/frame/FrameSerializer...`
       (used for MHTML today)
     * `//third_party/WebKit/Source/platform/mhtml/MHTMLArchive...`