When generating code, store the vector of protected instruction data
as unique_ptr. It only becomes a shared_ptr once ownership has been
transferred to the NativeModule, because it can be shared between
different instances of the same function.

Drive-by: Remove dead accessor in PipelineData.

R=ahaas@chromium.org

Change-Id: I7571b32bf89f3c816683c5a77ac08fe8c22eb968
Reviewed-on: https://chromium-review.googlesource.com/853496Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50404}