Prevent crash in Page::pluginData with --site-per-process.

The main frame may not be local if this is an OOPIF.

BUG=418708
TEST=See bug for repro steps.

Review URL: https://codereview.chromium.org/615653002

git-svn-id: svn://svn.chromium.org/blink/trunk@182939 bbb929c8-8fbe-4397-9dbb-9b2b20218538