Remove m_margins member from FrameView

This member is properly updated but it is never used. We can thus remove it
entirely, as well as the related code. This actually gets rid of a decent
chunk of code in rendering (including virtual function calls).

It decreases overall binary size by over 10Kb.

R=esprehn, adamk

Review URL: https://codereview.chromium.org/136793014

git-svn-id: svn://svn.chromium.org/blink/trunk@165168 bbb929c8-8fbe-4397-9dbb-9b2b20218538