[//gpu] ClientSI: Separate out construction with/without SIIHolder

The only construction path in which ClientSharedImage is not supplied an
SIIHolder is via ImportUnowned(). This CL changes that path to go
through a new constructor that doesn't have an SIIHolder param and
changes the other constructors to CHECK() that an SIIHolder was passed.

This paves the road for accessing the SIIHolder in SetTextureTarget()
invoked from those constructors to obtain the MacOS-specific texture
target.

Bug: 41494843
Change-Id: I3c2c48c7230d3bbbc2bdf8991d4fd611676f52b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402876


Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280109}