Separate LineBreaker from RenderBlockLineLayout

As part of my RenderBlockLineLayout refactoring campaign (tracked under
issue 316347) in this patch, I'm separating LineBreaker class into its
own file. Since BreakingContext::handleEndOfLine() implementation is not
inline, I moved it to BreakingContext.cpp in order to avoid linking errors.

BUG=336293

Review URL: https://codereview.chromium.org/142323003

git-svn-id: svn://svn.chromium.org/blink/trunk@165539 bbb929c8-8fbe-4397-9dbb-9b2b20218538