Let ChromeContentBrowserClient subclasses add extra_parts objects.

This allows embedders to customize this kind of startup operations
without editing chromium

Change-Id: Ic020b13a87b4e59d7156385da37555683d2418a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1944392
Commit-Queue: Yngve Pettersen <yngve@vivaldi.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721785}

chrome/browser/chrome_content_browser_client.h

@@ -658,6 +658,12 @@ class ChromeContentBrowserClient : public content::ContentBrowserClient {
                                  content::BrowserContext* browser_context);
   virtual ui::NativeTheme* GetWebTheme() const;  // For testing.
 
+  // Used by subclasses (e.g. implemented by downstream embedders) to add
+  // their own extra part objects.
+  void AddExtraPart(ChromeContentBrowserClientParts* part) {
+    extra_parts_.push_back(part);
+  }
+
  private:
   friend class DisableWebRtcEncryptionFlagTest;
   friend class InProcessBrowserTest;