Add ExtendSelectionAndDelete handler in WebPlugin

This ExtendSelectionAndDelete message should be routed to plugin for
supporting IME on browser_plugin etc.

This is in addition to
https://codereview.chromium.org/102963005
(Add supportsInputMethod() interface for WebPlugin)
to support IME in WebPlugin.

This corresponds to nona's review comment in
https://codereview.chromium.org/103403006/#msg14

BUG=235573
TEST=will be covered in content_browsertest
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/100193005

git-svn-id: svn://svn.chromium.org/blink/trunk@163839 bbb929c8-8fbe-4397-9dbb-9b2b20218538