Add quad HD resolution and refactor GetWebrtcPixelsBucket

Refactor GetWebrtcPixelsBucket and extract a function that exposes
the index, which will be used in performance logging of the API.

Also add 2560x1440 as an intermediate resolution between full HD and
4K since the gap between these two resolutions is significant. Before
this change 2560x1440 was treated as 1920x1080.

Bug: chromium:1187565
Change-Id: I811646414fd5ef3ba657687532073d417289fbca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3513232


Reviewed-by: Will Cassella <cassew@chromium.org>
Commit-Queue: Johannes Kron <kron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979730}