Update currentSrc just before HTMLImageElement.onload

During the spec discussions [1], we have come to the conclusion that currentSrc
should be updated only before onload/onerror are triggered.
This CL implements that behavior.

BUG=378188

[1] https://github.com/ResponsiveImagesCG/picture-element/issues/155

Review URL: https://codereview.chromium.org/304313004

git-svn-id: svn://svn.chromium.org/blink/trunk@175327 bbb929c8-8fbe-4397-9dbb-9b2b20218538