Refactoring LineBreaker and RenderTextInfo out of RenderBlock.h

LineBreaker and RenderTextInfo are defined in RenderBlock.h, but only
used in RenderBlockLineLayout.cpp. Moving them there (LineBreaker needs
to be a friend class of RenderBlock) in the first step in the process
of refactoring LineBreaker::NextSegmentBreak into more manageable
chunks.

Review URL: https://chromiumcodereview.appspot.com/23855004

git-svn-id: svn://svn.chromium.org/blink/trunk@157402 bbb929c8-8fbe-4397-9dbb-9b2b20218538