Refactoring: Extract firstLetterLength method from RenderBlock::createFirstLetterRenderer

BUG=none
Test=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23628005

git-svn-id: svn://svn.chromium.org/blink/trunk@157340 bbb929c8-8fbe-4397-9dbb-9b2b20218538