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This CL adds common test utility code generating and serializing
combinations of Trust Token API parameters
(https://github.com/wicg/trust-token-api). This is so that the same code
can parameterize
1. the integration tests in trust_token_parameters_browsertest.cc, which
verify that JS parameters' values are correctly reflected in the network
stack, and
2. the iframe attribute parsing unittests in the child CL, which confirm
that the JSON-encoded 'trusttoken' iframe attribute is parsed into the
correct internal representation of the parameters' values.

It also refactors a test currently written using URLLoaderInterceptor to
use URLLoaderMonitor, which is much simpler.

Bug: 1062396
Change-Id: I3a109e2d0aacf3914f67aa7ac34ad3c7c363eccb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2115778Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754721}