Moved addStylesheetByUrl to it's own section.

This was a suggestion by dcheng in https://codereview.chromium.org/309143002/. Added a // Navigation transitions ----- section and moved the function there.

BUG=

Review URL: https://codereview.chromium.org/324153003

git-svn-id: svn://svn.chromium.org/blink/trunk@175933 bbb929c8-8fbe-4397-9dbb-9b2b20218538