Remove PluginLoadObserver and related logic, it was only used for NPAPI.

This really, truly kills NPN_GetURL, NPN_GetURLNotify, NPN_PostURL, NPN_PostURLNotify, and NPN_URLRedirectResponse.

BUG=493212

Review URL: https://codereview.chromium.org/1426923007

Cr-Commit-Position: refs/heads/master@{#363119}
91 files changed