[Blink] Remove CHECKs on not using low-latency from WebGLRCBase

It's outside the scope of these methods to dictate that they're called
only when low-latency isn't used, and removing these CHECKs simplifies
reasoning about the body of CanUseDrawingBufferSIWithoutCopy() wrt the
callsites where it's actually meaningfully invoked.

Bug: 352263194
Change-Id: Iadda0b9a06793c5320c309b37e304e6836b73ef2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7013295
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526705}