docs: Change "ChromeOS" to "Chrome OS".

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 83 matching lines @@
           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` 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
+and Linux (it is the default on Chrome OS), 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":
 
 * `SavePackage` creates only a single `SaveItem` (always `SAVE_FILE_FROM_NET`)
   and asks `SaveFileManager` to process it
   (as in the Complete HTML individual SaveItem handling above.).
@@ skipping 16 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...`