Use const ref for Browser create_params()

TBR=ellyjones@chromium.org

Bug: 1012169
Change-Id: I66512bdab46358e96bbcf90e3fd056bc8468bdea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847704
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Julian Watson <juwa@google.com>
Cr-Commit-Position: refs/heads/master@{#704039}

chrome/browser/ui/browser.h

@@ -286,7 +286,7 @@ class Browser : public TabStripModelObserver,
 
   // Accessors ////////////////////////////////////////////////////////////////
 
-  CreateParams create_params() const { return create_params_; }
+  const CreateParams& create_params() const { return create_params_; }
   Type type() const { return type_; }
   const std::string& app_name() const { return app_name_; }
   bool is_trusted_source() const { return is_trusted_source_; }