This was causing a very large performance regression in Texture
re-binding. Instead pass by const &, which should be fine, since
values in std::map (and unordered_map) are not reallocated when the
map changes.

Also make the SharedResource type non-copyable, and add an explicit
makeCopy method for when we need to clone the shared pointer.

BUG=angleproject:2034
BUG=chromium:727318

Change-Id: I39508a6ca4b41e4da31fe68899caa4464138cada
Reviewed-on: https://chromium-review.googlesource.com/517670
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>