Check that SVGImage is loaded in ImageNotifyFinished()

After https://codereview.chromium.org/2613853002/, SVGImage should be
completely loaded when ImageNotifyFinished() is called.
This CL explicitly checks that assumption in ImageNotifyFinished().

This is a reland of https://codereview.chromium.org/1515963005/,
with a stronger assumption of load event completion.

BUG=382170, 690480, 667641

Review-Url: https://codereview.chromium.org/2888953004
Cr-Commit-Position: refs/heads/master@{#473608}
3 files changed