Use the correct v8 context for conversions when calling into the plugin from JS
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}