Add a separate warning message for topSites permission.

The current warning is the generic "Read your browsing history"
warning. The new warning for topSites will be suppressed by
read only and read/write history warnings, and all hosts warning.

BUG=404334

Review URL: https://codereview.chromium.org/482953003

Cr-Commit-Position: refs/heads/master@{#292545}
5 files changed