[FPF-CI] Block canvas readback when experimental flag is on

When the RuntimeEnabledFeature BlockCanvasReadback is enable, any type
of canvas readback will result in a DOM exception. The feature is
controlled through a browser-level feature:
fingerprinting_protection_interventions::features::kBlockReadback, which
will typically only be enabled in Incognito mode (unless overridden by
the feature parameter to enable in all browsing modes).

Bug: 442851605
Change-Id: Ie9ad799f5db157c9ffb3a734006d0c646cbc2f0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6926120
Reviewed-by: Zainab Rizvi <rizvis@google.com>
Auto-Submit: Tom Van Goethem <tov@chromium.org>
Reviewed-by: Mike Taylor <miketaylr@chromium.org>
Reviewed-by: John Kim <johnykim@google.com>
Reviewed-by: Jean-Philippe Gravel <jpgravel@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Tom Van Goethem <tov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1514873}