Prevent crash in Page::pluginData with --site-per-process.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@182939 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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