webkit: Delay to show MediaController when surface changed.

Summary:
When surface changed, delay to show MediaController after hide, in order
to ensure status of views.

Change-Id: Ibead708cf11659e6e7297fd48d80da0c249f1079
CRs-Fixed: 521797

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D450