Error event shouldn't fire when image is updated on environment changes.

When environment changes triggered an image load, the error event was
wrongfully triggered. This is the wrong behavior for Web compat and
it made related tests flaky.
This CL fixes that.

BUG=410268,411672

Review URL: https://codereview.chromium.org/654253003

git-svn-id: svn://svn.chromium.org/blink/trunk@184917 bbb929c8-8fbe-4397-9dbb-9b2b20218538