Move "paragraph" granularity related functions to VisibleUnitParagraph.cpp

This patch moves "paragraph" granularity related functions to
"VisibleUnitParagraph.cpp" for ease of adopting them to Layout NG.

This patch is similar to the patch[1] which move "line" granularity related
functions to "VisibleUnitLine.cpp"

[1] http://crrev.com/2914793002: Move "line" granularity related functions to
VisibleUnitLine.cpp

BUG=707656
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2923153002
Cr-Commit-Position: refs/heads/master@{#477498}
3 files changed