Extensions: Remove the intermediate PermissionMessageStrings interface
Switch all callers over to the new system (CoalescedPermissionMessages).
Also rename GetCoalescedPermissionMessages to GetPermissionMessages - it's the only way to get permission messages now.
[This is part 2 of operation "remove the old permission message system".]
TBRing trivial changes in ephemeral_app_launcher.cc and extension_install_dialog_view_browsertest.cc.
TBR=benwells@chromium.org
BUG=398257
Review URL: https://codereview.chromium.org/
1218373003
Cr-Commit-Position: refs/heads/master@{#341734}