Avoid crashing if MHTMLGenerationManager::JobFinished is called twice.

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
@@ skipping 70 matching lines @@
 
 ### MHTML
 
 Very high-level flow of saving a page as MHTML:
 
 * Step 1: `WebContents::GenerateMHTML` is called by either `SavePackage` (for
           Save-Page-As UI) or Extensions (via `chrome.pageCapture` extensions
           API) or by an embedder of `WebContents` (since this is public API of
           //content).
 
-* Step 2: `MHTMLGenerationManager` coordinates generation of the MHTML file
-          by sequentially (one-at-a-time) asking each frame to write its portion
-          of MHTML to a file handle.  Other classes (i.e. `SavePackage` and/or
-          `SaveFileManager`) are not used at this step at all.
+* Step 2: `MHTMLGenerationManager` creates a new instance of
+          `MHTMLGenerationManager::Job` that coordinates generation of
+          the MHTML file by sequentially (one-at-a-time) asking each
+          frame to write its portion of MHTML to a file handle.  Other
+          classes (i.e. `SavePackage` and/or `SaveFileManager`) are not
+          used at this step at all.
 
-* Step 3: When done `MHTMLGenerationManager` calls a completion callback
-          which in case of Save-Page-As will end up in
+* Step 3: When done `MHTMLGenerationManager` destroys
+          `MHTMLGenerationManager::Job` instance and calls a completion
+          callback which in case of Save-Page-As will end up in
           `SavePackage::OnMHTMLGenerated`.
 
 Note: MHTML format is by default disabled in Save-Page-As UI on Windows, MacOS
 and Linux (it is the default on ChromeOS), but for testing this can be easily
 changed using `--save-page-as-mhtml` command line switch.
 
 
 ### HTML Only
 
 Very high-level flow of saving a page as "HTML Only":
@@ skipping 20 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...`