Add following methods to ToolbarModelImpl to abstract the dependencies
on content::WebContents.

- GetURL(): fetches the URL to display, if it exists, or return false,

- ShouldDisplayURL(): returns whether the URL should be displayed in the
  toolbar or not (embedder specific)

- GetSecurityLevel(): returns the security level of the underlying page,
  independently of the editing status of the toolbar,

- GetSearchTerms(): extracts the search terms for the current page from
  the content::WebContents (implementation depends on the embedder),

- GetCertificate(): returns the certificate for the current page, if
  any, otherwise returns null.

Inject the constant content::kMaxURLDisplayChars through the constructor
of ToolbarModelImpl to allow sharing the code with iOS that cannot depend
on content.

TBR=dzhioev@chromium.org
BUG=582921

Review URL: https://codereview.chromium.org/1653013002

Cr-Commit-Position: refs/heads/master@{#373317}