Fold ImageLoader::updateFromElementIgnoringPreviousError into updateFromElement

This CL is just a refactor that should have no functional impact. It is done as ground work in order to later add yet a third mode to updateFromElement.

BUG=

Review URL: https://codereview.chromium.org/378163002

git-svn-id: svn://svn.chromium.org/blink/trunk@177787 bbb929c8-8fbe-4397-9dbb-9b2b20218538