Copy EditorClient.h and EditorClientImpl.{cpp,h} to SpellCheckerClient.h and SpellCheckerClientImpl.{cpp,h}

This patch is a part of introducing SpellCheckerClient and SpellCheckerClientImpl. Another patch will update GYP files, e.g. http://crrev.com/66503015

This patch copies following files to introduce SpellCheckerClient for ease of maintaining editing feature and spell check feature:
- core/page/EditorClient.h to core/page/SpellCheckerClient.h
- web/EditorClientImpl.{cpp,h} to web/SpellCheckerClientImpl.{cpp,h}

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/62803006

git-svn-id: svn://svn.chromium.org/blink/trunk@162074 bbb929c8-8fbe-4397-9dbb-9b2b20218538