Storing ImageHelper as a pointer allows the storage to be swapped or shared
with other objects.

BUG=angleproject:2668

Change-Id: I2e51f24737be59ffe9f472e9b0b592774a792cd1
Reviewed-on: https://chromium-review.googlesource.com/c/1409404
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>