GitLab

Previously we were using the plugin context when converting objects during calls
into the plugin from JS. As decided previously in https://codereview.chromium.org/555583003/
we should always use the caller's context in these circumstances.

This CL also separates the checks in PepperPluginInstanceImpl::GetContext to
help diagnose crbug.com/416311.

BUG=416311

Review URL: https://codereview.chromium.org/588083002

Cr-Commit-Position: refs/heads/master@{#297136}