Let notifyScriptLoader() handle already detached ScriptLoaders.

If a ScriptRunner has been disposed of already, allow ScriptLoaders
to notify of their failure without asserting.

R=haraken
BUG=570012

Review URL: https://codereview.chromium.org/1644483002

Cr-Commit-Position: refs/heads/master@{#371772}
2 files changed