Remove some linked_ptr from /content

- Remove linked_ptr from content/browser/browser_plugin/browser_plugin_guest.cc
- Remove unused linked_ptr.h includes.

BUG=556939
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://chromiumcodereview.appspot.com/2426423004
Cr-Commit-Position: refs/heads/master@{#426695}
7 files changed