When copying from a non-mappable NativeStorage to a system memory
based storage, a staging buffer storage should be used. Currently
this is only done for PackStorage.

This covers the missed SystemMemoryStorage and
EmulatedIndexedStorage. Also it adds a triggering test case to
expose the bug.

BUG=angleproject:2076

Change-Id: I278a0eef85751e966c1c48ddd71010092a14a3f7
Reviewed-on: https://chromium-review.googlesource.com/542595
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>