Remove plugin occlusion support from Blink.

This was used to support windowed plugins, but all supported
plugins are windowless now.

Bug: 493212
Change-Id: I4f0fbcb20b2ca1d042c99089108eb0db89834114
Reviewed-on: https://chromium-review.googlesource.com/500028
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#470434}
16 files changed