Previously we were just dumping any unexpected errors here. Fix this
by passing in the RT explicitly in the constructor, although we still
store the EGLImage pointer in case the RT changes.

BUG=angleproject:1455

Change-Id: Ie4d4a07c3a7b3a5ccc0a43c4e80bfc6ee43a676b
Reviewed-on: https://chromium-review.googlesource.com/365411Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>