Copy TextIerator.cpp to PlainTextRange.cpp and InputMethodController.h to PlainTextRange.cpp

This patch is a part of introducing PlainTextRange class in editing module. Another patch will update core.gypi to build PlainTextRange.

BUG=n/a
TEST=n/a; no behavior changes
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/34313003

git-svn-id: svn://svn.chromium.org/blink/trunk@160321 bbb929c8-8fbe-4397-9dbb-9b2b20218538