chore: cherry-pick 809231f0c9 from chromium.

[ppontes]

[privacy_budget] Remove unnecessary kCanvasReadback metrics.

The identifiability metrics recorded under kCanvasReadback surface type
used two conflicting sources as inputs: the CanvasRenderingContext
type, and the paint-op digest.

There are known collisions between resulting IdentifiableSurface values
from the two sources, which makes it impossible to losslessly separate
the two during analysis.

While the fact that a canvas readback happened is interesting, it
doesn't help determine the observed diversity of clients. Hence this
change removes one of those sources: the CanvasRenderingContext type.

M86 merge conflicts and resolution:

• third_party/blink/renderer/core/offscreencanvas/offscreen_canvas.cc
  M86 does not have the code removed in original CL.
• third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc
  third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc
  Removed corresponding code, kept old API.

(cherry picked from commit 809231f0c9fdc6180b6a99cf067d0a32db053034)

(cherry picked from commit b206b57b96985713ad167738f6839a8d32db78f2)

Bug: 1161379, 1186641
Change-Id: I770cb631c9c4afe4c36d1b129aaf61410db25d43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600386
Commit-Queue: Asanka Herath asanka@chromium.org
Reviewed-by: Caleb Raitto caraitto@chromium.org
Reviewed-by: Kentaro Hara haraken@chromium.org
Cr-Original-Original-Commit-Position: refs/heads/master@{#847480}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2785145
Reviewed-by: Justin Novosad junov@chromium.org
Reviewed-by: Juanmi Huertas juanmihd@chromium.org
Reviewed-by: Asanka Herath asanka@chromium.org
Commit-Queue: Yi Xu yiyix@chromium.org
Cr-Original-Commit-Position: refs/branch-heads/4389@{#1599}
Cr-Original-Branched-From: 9251c5db2b6d5a59fe4eac7aafa5fed37c139bb7-refs/heads/master@{#843830}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2794506
Reviewed-by: Artem Sumaneev asumaneev@google.com
Reviewed-by: Victor-Gabriel Savu vsavu@google.com
Auto-Submit: Artem Sumaneev asumaneev@google.com
Commit-Queue: Artem Sumaneev asumaneev@google.com
Cr-Commit-Position: refs/branch-heads/4240@{#1586}
Cr-Branched-From: f297677702651916bbf65e59c0d4bbd4ce57d1ee-refs/heads/master@{#800218}

Notes: Security: backported fix for 1161379, 1186641.

[release-clerk]

Release Notes Persisted

Security: backported fix for 1161379, 1186641.