mediaeditor: Fix c++11 narrowing

Summary: Depends on D309 D310

Test Plan: verified

Reviewers: randall.rushing, jriordan001

Reviewed By: randall.rushing

Differential Revision: http://review.codefirex.com/D312

Change-Id: I34a0e0e0fc9879e6c4a12a1dd8375dd409042878
Signed-off-by: DHOMD <DHO@dho.im>