This is follow up of crrev.com/c/2368038. There was concern that
initImmutableImage and initImage are a bit duplicated. This CL gets rid
of initImmutableImage and uses initImage instead.

Bug: b/181800403
Change-Id: I2c73c7ce979792cc762214f1e5ef9978eeab6212
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2800422
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>