Allow extensions to specify that they are not allowed in incognito mode.
Add "not_allowed" as a valid value to the "incognito" field in the
manifest. Track incognito behavior using an enum instead of a boolean
to distinguish between "split", "spanning", and "not_allowed". Add
CanBeIncognitoEnabled method to utils which checks the manifest entry
in addition to checking if extension is platform app or component.
BUG=455756
Review URL: https://codereview.chromium.org/
1351223003
Cr-Commit-Position: refs/heads/master@{#349806}