Copy SpellChecker.{cpp,h} to SpellCheckRequestManager.{cpp.h}

This patch copies SpellChecker.{cpp,h} to SpellCheckRequestManager.{cpp.h} for preparation of renaming SpellChecker class to SpellCheckRequestManager, to make class name more explicitly denotes what it does, and free its name to use name SpellChecker for decoupling spell checking and Editor class.

Note: Differences between original file and new file are for style checking in pre-submit.

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

Review URL: https://codereview.chromium.org/22732003

git-svn-id: svn://svn.chromium.org/blink/trunk@155831 bbb929c8-8fbe-4397-9dbb-9b2b20218538