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This CL expands the Fetch API, when the runtime-enabled feature
"TrustTokens" is enabled, to include a new experimental parameter
`trustTokens` denoting a request to execute a Trust Tokens protocol step
(https://github.com/wicg/trust-token-api) alongside the fetch at hand,
by adding request headers and processing corresponding response
headers.

This is an experimental interface that will be used in an origin trial.
It only needs to support fetches from frames (i.e., it is OK from a
usability perspective for the origin trial if the parameter is not
populated when fetch is called from other settings).

The main parts of the CL are:
- as specified in the linked explainer, expand Fetch's RequestInit
dictionary with a new trustToken parameter, specifying a single Trust
Tokens protocol operation of token issuance, token redemption, or
request signing;
- add this data to blink::ResourceRequest and associated objects, so
that it gets copied to network::ResourceRequest in
blink::PopulateResourceRequest.

Test:
- Adds integration tests in //content to ensure that calling the API
results in the right ResourceRequest members being populated.

Change-Id: I5c5b0c0ed5450eeb80c19be856f62941e50ec0e1
Bug: 1043118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036648Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749622}