Replace remaining direct access to m_imageElements with imageElements().

This is a follow-up of http://src.chromium.org/viewvc/blink?view=rev&rev=165719.

BUG=165719
TBR=keishi@chromium.org

Review URL: https://codereview.chromium.org/134733022

git-svn-id: svn://svn.chromium.org/blink/trunk@165722 bbb929c8-8fbe-4397-9dbb-9b2b20218538