We were leaking BufferHelper pointers in a couple cases. Fix these
systematically by using a unique_ptr template.

This leak was detected running ANGLE with LSAN enabled.

Bug: angleproject:5377
Change-Id: I30ab235105cf74fae2cfc62a22dde534e2d07e81
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2567641Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>