GitLab

Image:
- Adds move constructor/assignment. Not technically required as Image is
  cheaply copyable, but this allows more efficient moving without
  incrementing/decrementing the refcount. Also safer for multi-threaded
  usage.
- Removes SwapRepresentations. Was unused, and superseded by std::move.

ImageFamily:
- Removes copy constructor/assignment. ImageFamily can be fairly
  heavyweight, so it's best to move it rather than copying. Also it is
  heavily used across threads and accidental ImageFamily copying is
  responsible for data races (https://crbug.com/749342).
- Adds Clone method, an explicit copy operator for use if necessary.
- Adds move constructor/assignment. This should be used where possible.

BUG=600237,749342

Change-Id: I00482c8440d62edb8d9fb216c3de65bf4102d153
Reviewed-on: https://chromium-review.googlesource.com/588033Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490704}