GitLab

It is safer to hold a handle to v8 context in PepperTryCatchVar than to try
retrieving it again on destruction from the frame (through the plugin instance) which has
a chance of being torn down.

BUG=411445

Review URL: https://codereview.chromium.org/562763005

Cr-Commit-Position: refs/heads/master@{#294288}