[//cc] Eliminate GpuRasterBufferImpl reading Backing::overlay_candidate

It can just access client_->tile_overlay_candidate_ directly as it
does for client_->is_using_raw_draw_. The former field is set in the
constructor and not changed [1].

This is a step toward eliminating Backing::overlay_candidate entirely.

[1] https://source.chromium.org/chromium/chromium/src/+/main:cc/raster/gpu_raster_buffer_provider.cc;l=116;drc=29646f92766d515808be74c85bd5b511e032c023;bpv=1;bpt=1

Bug: 40064122
Change-Id: I955b1cfb82738df9031dc43a5a35f2af56b772f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6288532
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423786}