We can keep a shared reference to the Pipeline and Descriptor Set
layouts in the Program. This ensures they are not in use when they are
deleted. Note that they are allowed to be deleted as long as no command
buffers are currently recording with them. If the Program is deleted
then there should be no further commands using these layouts.

Bug: angleproject:2462
Change-Id: I75161b3ce1ee8eae33dd6becee79b4262b844cdd
Reviewed-on: https://chromium-review.googlesource.com/1089807
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>