Re-factor InputMethodController for setSelectionOffsets

This patch moves implementation of Editor::setSelectionOffsets related member functions in WebViewImpl to InputMethodController to make Editor and WebViewImple smaller and improve modularity of InputMethodController for ease of maintenance.

BUG=276777
TEST=n/a; no behavior changes, webkit_unit_tests covers.
R=kochi@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/26973003

git-svn-id: svn://svn.chromium.org/blink/trunk@159823 bbb929c8-8fbe-4397-9dbb-9b2b20218538