[//content] Streamline WebGraphics3DContextProviderImpl DCHECK

GetGrContext() always returns null, so the second part of the ||
statement here would crash if ever executed. We have shipped OOP-R, so
this code is never actually executed in practice. This CL streamlines
the DCHECK as part of the work of tearing down non-OOP-R code.

Bug: 391648152
Change-Id: Iaf235c8afb0dca17e11d67bc4ab2f9141e2acfb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6951235
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1516537}