[Blink] Simplify HTMLCanvasElement::UpdateMemoryUsage()

The two types of buffers being counted here have their memory usage
computed exactly the same way. Merge them into just one variable and
do the computation once. This simplifies a followup CL that will
eliminate CanvasRenderingContext::DrawsViaGpu().

Note: This entire computation is kind of wonky and should at some point
be rethought at a higher level, but that's outside the scope of my work
here.

Bug: 352263194
Change-Id: I92bc7482381f69b590c813d9eca06cff7ac8c231
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6632918
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1473437}