Add minimal Input Method interface for WebPlugin.

supportsInputMethod() will be used to determine whether
the plugin implementation can handle input method, and
setComposition/confirmComposition corresponds to the same
method in WebViewImpl.cpp, to handle composition text
updates.

This patch is required for Chromium side patch:
https://codereview.chromium.org/103403006/

BUG=235573

Review URL: https://codereview.chromium.org/102963005

git-svn-id: svn://svn.chromium.org/blink/trunk@163522 bbb929c8-8fbe-4397-9dbb-9b2b20218538