<webview>: Fix auto-permission rejection logic

loadplugin and other future permissions live entirely in the chrome layer.

There needs to be one upper bound across all permission types after which
permissions are auto-rejected without exposing them to the permission API.
This upper bound previously existed in the content layer. Since permissions
can originate in either layer, this CL moves the check to WebViewGuest
from BrowserPluginGuest.

BUG=none

Review URL: https://codereview.chromium.org/25519003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226554 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226554