Copy FrameSelection.{cpp,h} to Caret.{cpp.h}

This patch adds two files Caret.cpp and Caret.h as copy of FrameSelection.cpp and FrameSelction.h for preparation of moving code from FrameSelection.cpp to Caret.cpp to reduce size of FrameSeleciton.cpp for ease of maintenance.

Changes between Caret.{cpp,h} against FrameSelection.{cpp,h} are for passing pre-submit style check.

Caret.cpp isn't part of build until another patch changes core.gypi.

BUG=n/a
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/22547002

git-svn-id: svn://svn.chromium.org/blink/trunk@155667 bbb929c8-8fbe-4397-9dbb-9b2b20218538