Move SecurityLevel into a class of its own

Move SecurityLevel into a class of its own

This CL moves |GetSecurityLevelForWebContents| and the |SecurityLevel|
enum into a class of their own: |ConnectionSecurityHelper| in
//chrome/browser/ssl (since it is applying Chrome-specific security
policy to determine the security level of a page).

Later, we will use the new |ConnectionSecurityHelper| class to compute a
security level for a page to send to the DevTools Security
panel. Specifically, we need to be able to calculate the security level
for a given page, not just the WebContents for the active tab.

BUG=484392

Committed: https://crrev.com/ca7c0a3374c21ab10c821f4f3a890afc79182acf
Cr-Commit-Position: refs/heads/master@{#329591}

Review URL: https://codereview.chromium.org/1123943002

Cr-Commit-Position: refs/heads/master@{#329724}