Only flag the LayoutObject on CSP error if one is attached

When instantiating a plugin through the (somewhat special) code-path
that does not require a LayoutObject to be present, we would end up
dereferencing a null-pointer if a CSP error was flagged, failing the
plugin load sequence.

BUG=627694

Review-Url: https://codereview.chromium.org/2162473003
Cr-Commit-Position: refs/heads/master@{#406295}
3 files changed