Clear m_imageElements when it gets dirty.

Keeping out-of-date information is risky.

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/141413005

git-svn-id: svn://svn.chromium.org/blink/trunk@165825 bbb929c8-8fbe-4397-9dbb-9b2b20218538