Add UI for local manipulation of the PDF plugin preference.

It shows the state of the pref in the content settings dialog and allows
its modification if not controlled by policy in which case an idicator is
present to make this visible.

Known issues that will be addressed in future CLs

- On about:plugins the plugin is shown as disbled by policy even if disabled
  by the user.
- If Chrome is default app for PDFs and this setting is on a vicious loop
  will happen where Chrome will copy the file over and over. Downloads should
  not proceed to opening PDFs in this case regardless of the user's choice.

BUG=628014
TEST=automatic ui tests.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2392923002
Cr-Commit-Position: refs/heads/master@{#423268}
4 files changed