[//gpu] ClientSI: Remove usage of GetPlatformSpecificTextureTarget()

This CL inlines the values returned by
GetPlatformSpecificTextureTarget() [1] inside
ComputeTextureTargetForSharedImage() on all platforms other than Mac.
This provides a step on the path toward complete decoupling of
ClientSharedImage's `texture_target_` field from the legacy functions
that compute texture targets, which will enable getting rid of those
functions entirely once the codebase is definitively transitioned to
ClientSharedImage::GetTextureTarget().

Transition of the Mac call will follow; it is slightly more complex as
we need to transition to using
SharedImageCapabilities::macos_specific_texture_target.

[1] https://source.chromium.org/chromium/chromium/src/+/main:gpu/command_buffer/common/gpu_memory_buffer_support.cc;l=181-198?q=GetPlatformSpecificTextur&ss=chromium

Bug: 41494843
Change-Id: I4d9918d37c1ebffbb7776d8678b7d4615da16bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402490


Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280183}