[SavePage] Do not save images with errors

In this page,while saving the page if images with load error/decode error are saved,
while rendering the mhtml page the broken images are shown in place of those images.

BUG=346254
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/177243005

git-svn-id: svn://svn.chromium.org/blink/trunk@169616 bbb929c8-8fbe-4397-9dbb-9b2b20218538