Move RenderTextInfo and WordMeasurement into their own headers.

They don't really belong in their current headers, and it's confusing.
For example, RenderInfo is defined in LineBreaker.h and its constructor
and destructor are defined in RenderBlockLineLayout.cpp.

Review URL: https://codereview.chromium.org/329853006

git-svn-id: svn://svn.chromium.org/blink/trunk@176057 bbb929c8-8fbe-4397-9dbb-9b2b20218538