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Despite Google trying to steal the name for an OS, Chrome is the trim around the edges of any viewport. Since we have fixed terms for the Annunciator Row on app design, this is generally instead shorthand for "browser chrome" or the parts of the web browser that appear automatically.
On mobile browsers, there is usually only top chrome, consisting of a Masthead which contains the URL/search entry fields, a Revealable Menu and possibly other controls such as Back.
On mobile especially, these are conditional. Many browsers provide for full-screen views or hide the chrome as the user begins to scroll. This is beneficial to the end user's experience generally, but the user does have ultimate control and can exit full screen or recall the top navigation.
Always design browser experiences with the assumption Chrome is visible. Like designing for small screens, and for landscape as well as portrait, this helps assure your interface fits and can be used in all conditions.
If available, never force chrome-free browsing, but always allow the user to go back to a normal view where they can use their browser controls freely.